Suction line accumulator

Suction line accumulator

A suction line accumulator is considered mandatory on all systems 2 HP and larger in size, and is recommended for all units. The purpose of the accumulator is to intercept any liquid refrigerant which might flood through the system before it reaches the compressor, particularly on start-up or on hot gas defrost cycles. Because crankcase heaters or a pumpdown cycle are not always operative on transport units, the accumulator is the best protection that can be provided for the compresor. 

Provisions for positive oil return to the crankcase must be provided, but a direct gravit flow is not acceptable since this would allow liquid refrigerant to drain to the crankcase during shutdown periods. Capacity of the accumulator usually should be minimum of 50% of the system charge, but the required size will vary with system desing. Tests are recommended during the design phase of any new unit to determine the minimum capacity for proper compressor protection.

An external souece of heat is desirable to accelerante the boiling of the liquid refrigerant in the accumulator so that it may return to the compressor as gas. Mounting in the condenser air stream or near the compressor will normally be satisfactory.

Liquid line filter-drier & Head exchanger

Liquid line filter-drier

On all transport refrigeration systems, because of the uncertainties of installation and service, a liquid line filter-drier is essential. It is recommended that the filter-drier be aversized by at least 50% for the refrigerant charge because of the many opportunities during field maintenance for moisture to enter the system. It should have flare connections for easy replacement.

Head exchanger

A heat excharger should be considered mandatory on all units. It improves the performance, insures liquid refrigerant at the expansion valve, and helps assure the return of dry gas. Normally it should be located inside the refrigeranted space to avoid loss of capacity, but it can be located externally if insulated.

Purging of air from system

Purging of air from system

Occasionally due to improper installation or maintenance procedures, a unit will not be completely evacuated, or oir will be allowed to enter the system after evacuation. The non-condensable gases will exert their own pressures in addition to refrigerant pressure, and will result in head pressure considerably above the normal condensing pressure.

Aside from the loss of capacity resulting from the higher head pressure, the presence of air in the system will greatly increase the rate of corrosion and can lead to possible carbon formation, copper plating, and/or motor failure.

As a temporary measure, it may be possible to purge refrigerant from the top of the condenser while the unit is not operating, and blow out any air trapped in the condenser. However, it is almost impossible to purgue all of the air out of the compressor cranckase, and air may also trap in the receiver. If it is discovered that air has been allowed to contaminate the system, the refrigerant should be removed, and the entire unit completely evacuated with an efficient vaccum pump.

Liquid Line Filter-Drier

On all transport refrigeration systems. because of the uncertainties of installation and service, a liquid line filter-drier is essential. It is recommended that the filter-drier be oversized by at least 50% for the refrigerant charge because of the many opportunities during field maintenace for moisture to enter the system. It should have flare connections for easy replacement.

Receiver

Receiver 

Because of field installation and repair, all units should be equipped either with a receiver or an adequately sized condenser so that the refrigerant charge is not critical. Valves should be provided so that the system can be pumped down. A positive liquid level indicator on the receiver will oid in preventing over-charging, and high and low test cocks have been used satisfactorily fot this purpose. The sized of the receiver should be held to the minimum required for safe pump down.

It is recommended that a charging valve be provided in the liquid line. While not essential, it is a fact that most servicemen will charge liquid rather than vapor into a system, and a charging valve makes this possible without damage to the compressor.

On units in operation over-the-road, powered either from the truck engine or a separate engine power source, the receiver may be subjeted to temperatures higher than the condensing temperature because of heat given off by the engine. This can result in abnormally high condensing pressure because of liquid refrigerant being forced back into the condenser, excessive refrigerant charge requirements, and flashing of liquid refrigerant in the liquid line. If excessive heating of the receiver can occur, provisions should be made for ventilation of the receiver compartment with ambient air, or the receiver should be insulated.

Crankcase pressure regulating valve & Condenser

Crankcase pressure regulating valve

In order to limit load on the compressor, a crankcase pressure regulating valve may be necessary. During periods when the valve is throttling, it acts as a restrictor, and on start-up or during a hot gas defrost cycle, it acts as an expansion valve in the line. The preferred location for the CPR valve is ahead of the suction line acculator. The accumulator will trap liquid refrigerant feeding back and allow it to boil off or feed the compressor at a metered rate to avoid compressor damage. However, location of the accumulator ahead of the CPR valve is acceptable if the accumulator has adequate capacity to prevent liquid floodback to the compressor.

The CPR valve should be sized for a minimum pressure drop to avoid loss of capacity, and should never be set above the published operating range of the compressor.

Condenser

Condenser construction must be rigid and rugged, and the fin surface should be treated for corrosion resistance unless the metal is corrosion resistant. The area in which the condenser is mounted affects its desing. Condensers mounted on the skirt of a truck or beneath a trailer receive a great deal of road splash, while those mounted high on the nose of a truck or trailer are in a somewhat cleaner atmosphere. If the condenser is mounted beneath a trailer facing in the direction of travel, a mud guard should be provided. The type of tube and fin construction affects the allowable fin spacing, but in general, fin spacing of no more than 8 fins to the inch is recommended, although some manufacturers are now using fin spacing as high as 10 and 12 per inch.

Since the unit operate for extended periods when the vehicle is parked, ram air from the movement of the vehicle cannot be considered in designing for adequate air flow, but the condenser fan should be located so that the ram air effect aids rather than opposes condenser air flow. It also should be born in mind that often many trucks or trailers will be operating side by side at a loading dock, and the air flow pattern should be such that one unit will not discharge hot air directly into the intake of the unit the next vehicle.

Since the avaiable for condenser face area is limited in transport refrigeration application, the condenser tube circuiting should be designed for maximum efficiency.

Low head pressure during cold weather can result in lubrication failureof compressors. With trucks operating or parked outside or in unheated garages in the winter months, this condition can frequently occur. A decreasedpressure differential across the expansion valve will reduce the refrigerant flow, resulting in decreased refrigerant velocity and lower evaporator pressure, permitting oil to trap in the evaporator. Frequently the feed will be decreased to the point that short-cycling of the compressor results. The use of a reverse acting pressure control for cycling the condenser fan, or some other type of pressure stabilizing device to maintain reasonable head pressure is highly recommended.

Oil Charge & Oil pressure safety control

Oil Charge

Compressor leaving the copeland factory are charged with Sunusi-3G oil should be used without specific authorization from the copeland application ebgineering department. The napthenic base of the Suniso-3G oil has definite advarages over paraffinic oils because of less tendency to separate from the refrigerant at reduced temperatures.

Compressor are shipped with a generous supply of oil. However, the system may require additional oil depending on the refrigerant charge and system desing. After the unit stabilized at its normal operating conditions on the unitial run-in, additional oil should be added if necessary to maintain the oil level at the 3/4 full level of the sight glass in the compressor crankcase. The high oil level will provide a reserve for periods of erratic oil return.


Oil pressure safety control

A major percentage of all compressor failures are caused by lack of proper lubrication. Only ralely is the lack of lubrication actually due to a shortage of oil in the system or failure of the oiling system. More often the source of the lubrication failure may be refrigerant floodback, oil trapping in the coils, or excessive slugging on start up.

To prevent failures from all these causes, the copeland warranty requires that an approved manual reset type oil pressure safety control with a time delay of 120 seconds be used on all copelametic compressors having an oil pump. The control operates on the differential between oil pump pressure and crankcase pressure, and the time delay serves to avoid shut down during short fluctuations in oil pressure during start up. A non-adjustable control is strongly recommended, but if an adjustable type control is used, it must be set to cyt aut at a net differential pressure of 9 psig. Oil pressure safety controls are available with alarm circuits which are energized should the oil pressure safety control open the compressor control circuit.

Refrigerant Charge

Refrigerant Charge

Refrigerant R-12 is used in most transport systems at the present time, but R-502 is well suited fot low temperature applications, and its use is increasing. Since R-502 creates a greater power requirement for a given compressor displacement than R-12, the motor-compressor must be properly selected for the refrigerant to be used. Different expansion valves are required for each refrigerant, so the refrigerants are not interchangeable in a given system and should never be mixed. receivers for R-502 require higher maximum working pressure than those used with R-12, so normally it is not feasible to attempt to convert an existing R-12 unit fot the use of R-502.

The refrigerant charge should be held to the minimum required for satisfactory operation. An abnormally high refrigerant charge will create potential problems of liquid refrigerant migration, oil slugging, and loss of compressor lubrication due to bearing washout or excessive refrigerant foaming in the crankcase.

System should be charged with the minimum amount of refrigerant necessary to insure a liquid seal ahead of the expansion valve at normal operating temperatures. For an accurate indication of refrigerant charge, a sight glass is recommended at the expansion valve inlet, and a combination sight glass and moisture indicator is essential for easy field maintenance chexking. It should be born in mind that bubbles in the refrigerant sight glass can be caused by pressure drop or restrictions in the liquid line, as well as inadequate liquid subcooling. Manufacturer's published nominal working charge data should be used only as a genral guide, since each installation will vary in its charge requirements.

Refrigerant Migration

Refrigerant migration is a constant problem on transport units because of the varying temperatures to which the different parts of the system are exposed. On eutectic plate aplications, liquid refrigerant will be driven from the considensing unit to the plates during the day's operation, with the threat of floodback on start-up. On both plate and blower units not in operation, the body and evaporator inmmediately after operation will be colder than the condensing unit, causing migration to the evaporator . During daytime hours the body and evaporator will warm up, and because of body instasulation will remain much warmer than the compressor during the night hours when the ambient temperature falls, resilting in a pressure differential sufficient to drive the refrigerant to the compressor crankcase.

Excessive refrigerant in the compressor crankcase on star-up can cause slugging bearing washout, and loss of oil from the crankcase due to foaming. Dilution of oil with excessive refrigerant result in a drastic reduction of the lubricating ability of the oil. Adequate protective measures must be taken to keep migration difficulties at a minimum. Consideration shoul be given to keeping the refrigerant charge as low as possible, using a pump down cycle, use of a suction accumulator, and the use of a liquid line solenoid valve.

Compressor Operating Position & Compressor Drive

Compressor Operating Position

occacionally compressor failures will occur due to loss of lubrication caused by parking the truck on too steep a slope. The resulting tilt of the compressor may cause the oil level to fall below the pick-up point of the oil flinger or oil pump.

operation of the unit whilethe truck is parked on steep inclines should be avoided. if this is unavoidable, the compressor so that oil will tend to flow to the oil pick-up  point. Since this will vary on disfferent model compressor, and the individual parking arrangement will affect the direction of the compressor pitch, each aplication must be considered individually.

In  severe cases, consult eith the compressor manufacturer.


Compressor Drive

Direct drive from an engine, either gasoline or diesel, to a compressor requires very careful attention to the coupling design. Alignment between the engine drive shaft and the compressor crankshaft is critical both in parallel and angular planes. Even slight angular misalignment can cause repetitive compressor crankshaft breakage. Because of the sharp impulses from the engine firing, a flexible coupling giving some resiliency is required. The coupling should be capable of compensating for slight parallel or angular misalignment and should also allow some slight endplay movement of the crankshafts. Nylon splines, neoprene bushings, and flexible disc type couplings have all been used successfully.

For a compressor driven from a power takeoff by means of a shaft and two iniversal joints, the crosses in the U-joints must be kept parallel to each other. Where possible, the compressor rotation should be in the same direction whether on electric standby or driven from the engine.

In driving a compressor with V-belts, care must be taken to avoid excessive belt tension and belt slap. A means for easily asjusting belt tension should be provided. It may be necessary to provide an idler pulley to dampen belt movement an long belt drives. Care should be taken to mount the compressor so that the compressor ahaft is parallel with the engine crankshaft.

Compressor Speed

Compressor Speed

Open type compressors operating from a truck engine by means of a power take-off or by a belt drive are subject to extreme speed ranges. A typical truck engine may idle at 500 RPM, to 700 RPM, run at 30 MPH and run at 3,600 RPM to 4,000 RPM over the highway at high speeds. Whatever the power take-off or belt ratio, this means the compressor must operate through a speed ratio range of 6 to 1 or greater unless it is disconnected from the power source by some means.

The compressor speed must be kept within safe limits to avoid loss of lubrication and physical damage. Operation within the phisical limitations of the compressor may be possible, for example from 400 RPM to 2,400 RPM. lt may be possible to use a cut-out switch to disconnect the compressor from the power source at given speed. The compressor manufacturer should be contacted for minimum and maximum and speeds of specific compressors.

If the compressor is of the accessible-hermetic type, there is no problem concerning speed so long as the electrical source is operating atthe voltage and frequency for which the motor was in order to abtain variable speed operation, the voltage and frequency on the normal alternating current generator will vary proportionally. Since the compressor speed and motor load will vary directly with the frequency, it is often possible to operate over a wide speed range with satisfactory results.

However, it should be born in mind that increasing the frequency and voltage of the generator above the level for which the compressor motor was designed will increase the load on the compressor, may overload the motor, and can result in bearing or other compressor damage. Operation at speesd too low to provide adequate compressor lubrication must also be avoided, although normally lubrication can be maintained on Copelametic compressors down to 600 RPM and possibly lower speeds.

Each new application involving operation of the compressor at a voltage and frequency differing from its nameplate rating should be submitted to the copeland aplication engineering departament fot approval.

One other problem that may arise with operation fron a variable speed generator is the operation of electrical contactors, relays, etc. on voltages below or above their nameplate rating. field tests have shown that the winding design and physical construction of electrical components can cause wide variation in voltage tolerance. The drop-out voltage of various types  of commercially available 220 volt contctors may vary from 145 volts to 180 volts depending on contruction. If it is plannedto operate at variable voltage and frequencies, the electrical components which are to be used should be extensively tested at the electrical extremes in cooperation with the manufacturer to insure proper operation.

Transport Refrigeration & Compressor cooling

Transport Refrigeration

Truck and trailer refrigeration is an increasingly important segnent of the refrigeration industry. Despite the fact that transport applications face many iperating problems peculiar to theyr unage, there exists very little aplication data pertaining to this field.

Many compressor failures in transport refrigeration unage are the result of system malfunction rather than the result of mechanical wear. It is clear that substantial savings in operating cost, and tremendous improvements in unit perfonmance and life would be possible if the causes of compressor failure could be removed. Primarily the problem boils down to one of making sure that the compressor has adequate lubrication at all times.

Part of the problem of identifying the cause of failure stems from the fact that far too few users realize that ultimate failure of a compressor resulting from lack of lubrication frequently takes place at a time when there is an adequate supply of oil in the crankcase. This is due to continued deterioration of the moving parts resulting from the original or repeated damage in the past. It is not uncommon for a damaged compressor to operate satisfactorily all winter and then fail in the spring when subjeted to heavier loads.

Another source of field problems is the fact that many units are installed by personnel who may not have adequate training, equipment, or experience. Often units, particularly those in common carrier service, may be serviced in emergencies by servicemen not familiar with the unit, or indeed, with transport refrigeration generally.

Because of the intallation and service hazards, it is extremely important that the unit be properly designed and applied to minimize, and if possible, prevent service problems.


Compressor cooling

Air-cooled motor-compressors must have a sufficient quantity of air passing aver the compressor body fot motor cooling. Refrigerant-cooled motor-compressors are cooled adequately by the refrigerant vapor at evaporating temperatures above 0° F. saturation, but at evaporating temperatures below 0° F. additional motor cooling by means of air flow in necessary.

Normally the condenser fan if located so that it discharges on the compressor will provide satisfactory cooling. For proper cooling, the fan must discharge air directly against the compressor. The compressor cannot be adequately cooled by air pulled through a compartment in which the compressor is located. If the compresor is not located in the condenser discharge air stream, adequate air circulation must be provided by an auxiliary fan.

Basic principles of refrigeration piping design

Basic principles of refrigeration piping design

The desing of refrigeration piping systems is a continuous series of compremises. It is desirable to have maximum capacity, minimum cost, proper oil return, minimum-power consumption, minimum refrigerant charge, low noise level, proper liquid refrigerant charge, low noise level, proper liquid refrigetant control, and perfect flexibility of sistem operation from 0 to 100% of system capacity without lubrication problems. obviously all of these goals cannot be satisfied, since some are in direct conflict. In order to make an intelligent decision as to just what type of compromise is desirable, it is essential that the piping designer clearly understand the basic effects on system perfonmance of the piping design in the different parts of the system.

In general, pressure drop in refrigerant lines tends to decrease capacity and increase power requirements, and excessive pressure drops should be avoided. the magnitude of the pressure drop allowable varies depending on the particular segment of piping involved, and each part of the system must be considered separately. There are probably more tables and charts available covering line pressure drop and refrigerant line capacities at a given pressure drop than on any other single subject in the field on refrigeration.

It is most important, however, that the piping designer realize that pressure drop is not the only criteria that must be considered in sizing refrigerant lines, and that often refrigerant velocities rather thanpressure drop must be the determining factor in system desing. In addition to the critical nature of oil return, there is no better invitation to system difficulties than an excessive refrigerant charge.  A reasonable pressure drop is far more preferable than oversizer lines which can contain refrigerant far in excess of the system's needs. An excessive refrigerant charge can result in serious problems of liquid refrigerant contro, and the flywheel effect of large quantities of liquid refrigerant in the low pressure side of the system can result in erratic operation of the refrigerant contron divices.

The size of the service valve supplied on a compressor, or the size of the connection on a condenser, evaporator, accumulator, or other accessory does not determine the size of line to be used. Manufacturs select a valve size or connection fitting on the basis of its application to an evarge system, and such factors as the type of system control, variation in load, and other factors can be major factors in determining the poper line size may be either smaller or larger than the fittings on various system components. In such cases, reducing fittings must be used.

Since oil must pass throug the compressor cylinders to provide lubrication, a small amount of oil is always circulating with the refrigerant. Refrigeration oils are soluble in liquid refrigerant, and at normal room temperatures they will mix completely. Oil and refrigerant vapor, however, do not mix readily, and the oil can be properly circulated through the system only if the mass velocity of the refrigerant velocities must be maintained not only in the suction and discharge lines, but in the evaporator circuits as well.

Several factors combine to make oil return most critical at low evaporating temperatures. As the suction pressure decreases and the refrigerant vapor becomes less dense, the more difficult it becomes to sweep the oil along. At the same time as the suction pressure falls, the compression ratio increases, and as a result compressor capacity is reduced, and the weight of refrigerant circulated decreases. Refrigerantion oil alone becomes the consistency of melasses at temperatures below 0° F., but so long as it is mixed with sufficient liquid refrigerant, it flows freely. As the percentage of oil in the mixture increases, the viscosity increases.

At low temperature conditions all of these factors start to converge, and can create a critical condition. The density of the gas decreases, the mass velocity flow decreases, the as a result more oil starts accumulating in the evaporator. As the oil and refrigerant mixture becomes more viscous, at some point oil may start logging in the evaporator rather than returning to the compressor, resulting in wide variations in the compressor crankcase oil level in poorly designed systems.

Oil logging can be minimized adequate velocities and properly designed evaporators even at extremely low evaporating temperatures, but normally oil separators are necessary for operation at evaporating temperatures below -50° F. in order to minimize the amount of oil in circulation.

Refrigeration Piping

Refrigeration Piping 

probably the  first skill that any refrigeration apprentice mechanic learns is to make a soldered joint, and running piping is so common a proper performance of a system is overlooked. It would seem elementary  in any piping system  that what goes in one end of a pipe  must come out the other, but on a system with  improper piping, it is not uncommmon  for a serviceman  to add gallons of oil to a system, and it may seemingly disappear without a trace. It is of course lying on the bottom of the tubing in  the  system, usually in the evaporator or suction line. When the piping og operating condition is corrected, theoil will return and those same gallons of oil must be removed.

Refrigeration piping involver extremely complex  relationships in the flow  of refrigerant and oil.Fiuid flow is the name given in mechanical engineering to the study of the flow of any fluid, whether it might be a gas or a liquid, and the inter-relationship of velocity, pressure, friction, density, viscosity, and the work required to cause the flow.these relationships evolve into long mathematical equations which form the basis for the fan laws which govern fan performance, and the pressure drop tables for flow through  piping. But 99% of the theories in fluid flow  textbooks deal with the flow of one homogenous fluid, adn there is seldom even a mention of a combination flow of liquid, gas, and oil such as occurs in any refrigeration system. Because of its changing nature, such  flow mathematical equation, and practically the entire working knowledge of refrigeration piping is based on practical experience and test data.As a result, the general type of  gas and  liquid  flow that must  be maintained to avoid problems is known, but seldom is there one exact answer to any problem.

Interconnected systems

Interconnected systems 

When the crankcases of two or more compresors are interconnected for parallel operation on a single refrigeration system, serious problems of oil return and vibration may be encountered unless the system is properly designed.  The tandem compressors with an interconnecting housing replacing the individual stator covers provides a simple, trouble free solution to this problem.

Because of the potential operating ploblems, interconnection of individual compressors is not approved with the exception of factory designed, tested, and assambled units specifically aproved by the copeland application Engineering Department.

High And Low Pressure Controls

High And Low Pressure Controls

Both high and low pressure controls are recommended for good system desing on all air cooled systems 1 HP and larger, and are essential on all field installed air cooled systems and on all water cooled systems.

When used for low temperature unit operation control,  the low pressure control must not be set below the minimum operating limits of the compressor or the system.  One of the most frequent causes of motor overheating and inadequate lubrication is operation of the compressor at axcessively low suction pressures. Copeland specification sheets list the approved compressor operating range, and recommended minimum low pressure control settings for various operating ranges are shown in table 21.

High pressure controls may be either manual or automatic reset as desired by the customer. If of the manual reset type,  provision must be made to prevent liquid refrigerant flooding through the system to the compressor in the event of a trip of the high pressure control. 

Internal automatic reset pressure relief valves (copelimit)  are provided in most copelaweld compressors 1 1/3 HP and large. On factory assembled package systems, the internal copelimit valve may satisfy U. L. and code requirements without the use of an external high pressure control.  A similar high side to low side automatic reset pressure relief valve is installed in all copelametic compressors with displacements of 3,000 CFH or greater.

On factory assembled and charged package systems,  such as room air conditioner,  where loss of charge proection is not considered critical,  or where the motor protection device can provide loss of charhe prote tion, low pressure controls may not be essential although recommended.

Thermostatic Expansion Valves

Thermostatic Expansion Valves

Thermostatic expansion valves must be selected and applied in accordance with the manufacturer´s instructions. Either internally equalized or externally equalized type, the external equalizerline must be connected, preferably at a point beyond the expansion valve thermal bulb. Do not cap or plug the external equalizer connection as the valve will not operate without this connection.

Valve superheat should be preset by the valve manufacturer, and field asjustment ahould be set to provide 5º F. to 10º F. superheat at the thermal bulb location. Too high a superheat setting will result in starving the evaporator, and can cause poor oil return. Too low a superheat setting will permit liquid floodback to the compressor.

A minimum of 15º F. superheat at the compressor must be maintained at all times to insure the return of dri gas to the compressor suction chamber, and a minimum of 20º F. superheat is recommended. Note that this is not superheat at the expansion valve, but should be calculated from pressure measured at the suction service valve and the temperature measured 18´´ from the compressor on the bottom of a horizontal run of suction line tobing. Lower superheat can result in liquid refrigerant flooding back to the compressor during variation in the evaporator feed with possible compressor damage as a tinually returing to the compressor wear, as well as resulting in a loss of capacity.

It is important that users realize that flash gas in the liquid line can seriously affect expansion valve control. So long as a head of pure liquid refrigerant is maintained at the expancionvalve, its perfonmance is relatively stable. but if flash gas is mixed with liquid refrigerant fed to the valve, a larger orifice opening is required to feed the some weight of liquid refrigerat. The onlu way the orifice opening can be increased is by an increase in superheat, and as the persentage of flash gas increases, the superheat increases, the valve opens wider, and the evaporator is progressively more starved.

If the valve has been operating with a large percentage of flash gas entering the expansion valve, and a head of pure liquid refrigerant is suddenly restored, the orifice opening will be larger than required for the load, and liquid will flood throung the system to the compressor until the valve again regains control. Conventional expansion valves with the thermal bulb strapped to the suctionline may be somewhat sluggish in response, and it may be several minutes before control can be restored to normal.

Tipically, changes in the qualit of liquid refrigent feeding the expansion valve can occur quickly and frequently because of the action of head pressure control devices, sudden changes in the refrigeration load, hunting of the expansion valve, action of an unloading valve, or rapid changes in condensing pressure.

On systems with short suction lines and low superheat requirements, quick response thermal bulbs or wells in the suction line may be essential to avoid periodic floodback to the compressor.

Temperatures and pressure alone may not give a true picture of the actual liquid refrigerant control in a system. Excessive oil circulation has the effect of increasing the evaporating temperature of the refrigerant. The response of the expansion valve is based on the saturation pressure and temperature of pure refrigerant. In an operating system, the changed pressure-temperature characteristicof the oil rich refrigerant will give the expansion valve a false reading of the actual seperheat, and can result in a somewhat lower actual supeheat than apparently exists, causing excessive liquid refrigerant floodback to the compressor. the only real cure for this condition is to reduce oil circulation to a minimum. Normally excessive oil in the evaporator can only result from an excessive system oil charge or other factors which could cause excessiveoilcirculation, or from low velocities in the evaporator which result in oil logging. In low temperature applications where proper oil circulation cannot be maintained, an oil separator may be required.

Vapor charged valves are sastifactory for air conditioning usage, and are desirable in many cases because of their inherent pressure limiting characteristic. For all refrigeration applications, liquid charge valves should be used to prevent condensation of the charge in the head of the valve and the resulting loss of control in the event the head becomes colder than the thermal bulb.

A pressure limiting type valve may be helpful in limiting the compressor load, and also prevents excessive liquid refrigerant floodback on star-up on systems using hot gas defrost, the defrost load is normally greater than the refrigeration load, and some other means of limiting the compressor power input must be used if required.

The thermostatic expansionvalve must be sezed properly for the load. Although a given valve normally has a wide operating capacity range, excessively indersized or oversized valves can cause system malfuctions. Undersized valves may starve the evaporator, and the resulting excessive superheat may adversely affect the system perfonmance. Oversized valves can cause hunting, alternately starving and flooding the evaporator, resulting in extreme fluctuations in suction pressure.

Suction Line Filters

Suction Line Filters

A heavy duty suction line filter is recommended for every field installation. The filter will effectively remove contaminants from the system at the time of installation,  and serves to keep the compressor free of impurities during aperation. In the event of a motor burn, the filter will prevent contamination from spreading into the system through the suction line.

The suction line filter should be selected for a reasonable pressure drop, and should be equiped with a pressure ditting just ahead of the filter, preferably in the shell,  to facilitate checking pressure drop across the filter during operation.

Evaporators

Evaporators

Evaporators must be properly selected for the regrigeration load. Too large on evaporator might result in low velocities and possible oil logging. To small an evaporator will have excessive temperature differentials between the evaporating refrigerant and the medium to be cooled. The allowable TD between the entering air and the evaporating refrigerant may also be dictated by the humidity control required.

Internal volime of the evaporator turbing should be at a minimum to keep the system rwfrigerant charge a low as possible, so the smallest diameter tubing that will give acceptable performance should be used. Since pressure drop at low evaporating temperatures is critical for as capacity is concerned, multiple refrigerant ciucuits with fairly short runs are preffered. At the same time, it is essential that velocities of refrigerant in the evaporator be high enoungh to avoid oil trapping.

Vertical headers should have a bottom outlet to allow gravity oil drainage.

Heat Exchanger

Heat Exchanger

A liquid to suction heat exchanger is highly recommended on all refrigeration systems, and is required on packagge water chillers and water to water heat pumps because of the low operating superheat. On medium and low temperatureapplications, a heat exchanger increases system capacity, helps to eliminate flashing of liquid refrigerant head of the expansion valve, and aids both in preventing condensation on suction lines and in evaporating any liquid flooding through the evaporator.

On smail systems, soldering the liquid and suction lines together for several feet nakes ab effective heat exchanger.

Liquid Line Solenoid Valve

Liquid Line Solenoid Valve

A liquid line solenoid valve is recommended on all field installed systems with large refrigerant charges, particularly when the system has a charge in excess of three pounds of refrigerant per motor HP. The solenoid valve will prevent continued feed to the evaporator through the expansion  valve or capillary tube when the compressor is not operating, and will control migration of liquid refrigerant from the receiver and condenser to the evaporator and compressor crankcase.

If a pumpdown cycles is not used, the liquid line solenoid valve should be wired to the compressor motor terminals so that the valve will be de-energized when the motor is not operating.

Sight Glass And Moisture Indicator

Sight Glass And Moisture Indicator 

A combination sight glass and moisture indicator is essential for easy field maintenance on any system, and is required on any field installed system unless some other means of checking the refrigerant charge is provided.

A sight glass in a convenient means of determining the refrigerant charge, showing bubbles when there is insufficient charge, and a solid clear glass when there is sufficient charge. however, the operator should bear in mind that under some circumstances even when the receiver outlet has a liquid seal, bubbles or flash gas may show in the sight glass. This may be due to a restriction or excessive presure drop in the receiver outlet valve, a partially plugged drier or strainer, or other restriction in the liquid line ahead of the sight glass. If the expansion valve feed is erratic or surging, the increased flow when the expansion valve is wide open can create sufficient pressure drop to cause flashing at the receiver outlet.

Another source of flashing in the sight glass may be rapid fluctuations in compressor discharge pressure. For example, in a temperature controlled room, the sudden opening of shutters or the cyclinng of a fan can easuly cause a reduction in the condensing temperature of 10 º F. to 15 º F. Any liquid in the receiver may then be at a temperature higher than the saturated temperature equivalent to the lower condensing pressure, and flashing will continue until the system has stabilized at the new condensing temperature.

While the sight glass can be a valuable aid in servicing a refrigeration or air conditioning system, a more positive liquid indicator is desirable, and the system performance must be carefully analyzed before placing full reliance on the sight glass as a positive indicator of the system charge.

Liquid Line Filter-Drier

Liquid Line Filter-Drier

A loquid line filter-drier must be used on all field installed systems, and on all systems opened in the field for service. Filter-driers are highly recommended forall systems, but are not mandatory on factory assembled and charged units where careful dehydration and evacuation is possible during manufacture. Precharged systems with quick connect fittings having a rupture disc are considered to be the equivalent of factory charged systems.

Moisture can be a factor in many forms of system damage, and the reduction of moisture to an acceptable level van greatly extend compressor life and slow down harmful reactions. The dessicant used must be capable of removing moisture to a low end point and further should be of a type which can remove a reasonable quantity of acid. It is most important that the filter-drier be equipped with an excellent filter to prevent circulation of carbon and foreing particles.

Low Ambient Head Pressure Control

Low Ambient Head Pressure Control

within the operating limitations of the system, it ir desirable to take advantage os lower condesing temperatures whenever possible for increased capacity, lower discharge temperatures, and lower power requirements. However, too low a discharge pressure can produce serious malfuntions. Since the capacity af capillary tubes and expansion valves is proportional to the differential pressure across the capillary tube or valve, a reduction in discharge pressure will reduce its capacity and produce a drop in evaporating pressure.

Low discharge pressure can result in starving the evaporator coil with resulting oil logging, short cycling on low pressure controls, reduction of system capacity, or erratic expansion valve operation.

Systems with water cooled condenserd and cooling towers require water regulating valves, or some other means of controlling the temperature or the quantity of water passing through the condenser.

If air cooled air conditioning systems are required to operate in ambient temperatures below 60º F., a suitable means of controlling head pressure must be provided. Refrigeration systems are also vulnerable to damage from low head pressure conditions, and adequate head pressure controls should be provided for operation in ambient temperatures below 50º F.

Several proprietary control systems are available for low ambient operation, most of which maintain head pressure above a preset minimum by partially flooding the condenser and thus reducing the effective surface area. Methods of this type can control pressure effectively, but do require a considerable increase in refrigerant charge and adequate receiver capacity must be provided.

Air volume dampers on the condenser operated from refrigerant discharge pressure provide a simple, economical, and effective means of control which is widely used.

Adequate protection a lowest cost can often be provided by a reverse acting high presure control which senses discharge pressure, and acts to disconnec the condenser fan circuit when the head pressure falls below the control´s minimum setting. The proper adjustment of the off-on differential is particularly important to avoid excessive fan motor cycling, and the resulting fluctuations in discharge pressure may contribute to uneven expansion valve feeding. In cold ambient temperatures the condenser must be shielded from the wind.

Crankcase Pressure Regulating Valves

Crankcase Pressure Regulating Valves

In orden to limit the power requirement of the compressor to the allowable operating limit, a crankcase pressure regulating valve may be necessary. This most frequently occurs on low temperature compressors where the power requirement during pulldown periods or after defrost may be greatly in excess of the compressor motor's capabilities. Copeland compressors should not be operated at suction pressure in excess of the published limits on compressor specification sheets without approval of the copeland application Engineering Department.

Since any pressure drop in the compressor suctio line lowers the system capacity, the CPR valve should be sized for a minimum pressure drop. In order to restric pull down capacity as little as possible, the valve setting should be as high as the motor power requirement will allow.

thermal expancsion valves of the pressure limiting type are not recommended when a CPR valve is used, particularly if the pressure settings are fairly close. particularly if the pressure settings are fairly close, because of the possibility of the action of the two valves coming in conflict in their response to system pressures.

Crankcase Heaters

Crankcase Heaters 

On some systems operating requierements,  noise considerations, or customer preference may make the use of a pumpdown system undesirable, and crankcase heaters are frequently used to control migration.

By warming the oil, absorption of refrigerant by the oil is minimized, and under mild weather conditions, any liquid refrigerant in the crankcase can be vaporized and forced out of the compressor. For effective protection, heaters must be energized continuously, independent of compressor operation. Improperly sized heaters can overheat the oil, and heaters used on copeland compressors must be specifically approved by the copeland application Engineering Department.

It would be a mistake to assume that crankcase heaters are a dependable cure for all migration problems. As the ambient conditions contributing to migration worsen, the ability of the crankcase heater to keep refrigerant out of the crankcase decreases. If the suction line slopes toward the compressor, and the temperature to which the suction line is exposed is suffiently lower than the temperature of the oil, refrigerant may condenser in the suction line and flow back to the compressor by gravity at a rate sufficient to offset the heat introduced by the heater. Heaters will not protect against liquid slugs or excessive liquid flooding. However, where operating conditions are not too severe, crankcase heaters can provide satisfavtory protection against migration.

Where a pumpdown cycle is not used, crankcase heaters are mondatory on heat pumps, and on other air conditioning applications if the refrigerant charhe exceeds the established copeland limits, unless tests prove the compressor is adequately protected by other means.

To prevent possible damage in shipment, crankcase heaters are not installed on compressors at the factory.

Pumpdown System Control

Pumpdown System Control

Refrigerant vapor will always migrate to the coldest part of the system, and if the compressor crankcase can bacome colder than other parts of the system,  refrigerant in the condenser, receiver,  and evaporator will vaporize,  travel througn the system, and condense in the compressor crankcase.

Because of the difference in vapor pressure of oil and refrigerant, refrigerant vapor is attracted to refrigeration oil,  and even though no pressure or temperature difference exists to cause a flow,  refrigerant vapor will migrate through the system and condense in the oil until the oil is saturated. During off cycles extending several hours or more, it is possible for liquid refegerant to almost completely fill the compressor crankcase due to the oil attraction. For example in a system using R-12 refrigerant which is allowed to equalize at an ambient temperature of 70° ., the oil-refrigerant mixture in the crankcase will end up about 70% refrigerant before equilibrium is reached.

The most positive and dependable means of keeping refrigerant out of the compressor crankcase is the use of a pumpdown cycle. By closing a liquid line solenoid valve, the refrigerant can be pumped into the condenser and receiver, and the compressor operation controlled by means of a low pressure control. The refrigerant can thus be isolated during periods when the compressor is not in operation, and migration of refrigerant to the compressor crankcase is prevented.

Pumpdown control can be used on all thermostatic expansion valve systems with the addition of a liquid line solenoid valve, provided adequate receiver capacity is available. Slight refrigerant leakage may occur through the solenoid valve, causing the suction pressure to rise gradually, and a recycling type control is recommended to repead the pumpdown cycle as required. The accasional short cycle usually is not objectionable.

A pumpdown cycle is highly recommended whenever it can be used. If a not-recycling pumpdown circuit is required, then consideration should be given to the pumpdown for more dependable compressor protection.

Suction Line Accumulators

Suction Line Accumulators

If liquid refrigerant is allowed to flod through a refrigeration or air conditioning system ans return to the compressor before being evaporated, it may cause damage to the compressor due to liquid slugging, loss of oil from the crankcase, or bearing washout. To protect against this condition on systems vulnerable to liquid damage a suction accumulator may be necessary.

The accumulator´s funtion is to intercept liquid refrigerant before it can reach the compressor valves or crankcase. It should be located in the cuction line near the compressor, and if a reversing valve is used in the system, the accumulator must be located between the reversing valve and the compressor. Provisions for positive oil return to the crankcase must be provided, but a direct gravity flow which will allow liquid refrigerant to drain to the crankcase during shut-down periods must be avoided. The liquid refrigerant must be metered back to the compressor during operation at a controlled rate to avoid damage to the compressor.

Some systems, because of their design, will periodically flood the compressor with liquid refrigerant. Typically, this can occur on heat pumps at the time the cycle is switched from cooling to heating, or from heating to cooling. The coil which has been serving as the condenser ir partially filled with liquid refrigerant, and when suddenly exposed to suction pressure, the liquid is dumped into the suction line. On heat pumps equipped with expansion valves, there may be further flooding due to the inability of the expansion valve to effectively control refrigerant feed for a short period after the cycle change until the system operation is again stabilized.

A similar situation can accur during defrost cycles. With hot gas defrost, when the defrost cycles is initiated, the sudden introduction of high pressure gas into the evaporator may force the liquid refrigerant in the evaporator into the suction line. If the defrost cycle is such that the evaporator can fill with condensed liquid during defrost, or on systems utilizing electric defrost without a pumpdown cycle, an equally dangerous situation may exist at the termination of the defrost cycle.

On systems with a large refrigerant charge, or on any system where liquid floodback is likely to accur, a suction line accumulator is strongly recommended. On heat pumps, truck applications, and on any system where liquid slugging can occur during operation, a suction line accumulator is mandatory for compressor protection unless otherwise approved by the copeland application engineering department. The actual refrigerant holding capacity needed for a given accumulator is governed by the requirements of the particular application, and the accumulator should be selected to hold the maximum liquid floodback anticipated.

Oil Separators

Oil Separators

Proper refrigerant piping desing and operation of the system within its design limits so that adequate refrigerant velocities can be maintained are the only cure for oil logging problems, but an oil separator may be a definite aid in maintaining lubrication where oil return problems are particulary acute.

For examples, consider a compressor having an oil charge of 150 ounces, with the normal oil circulation rate being 2 ounces per minute. This means that on a normal system with proper oil return at stabilized conditions, two ounces of oil leave the compressor throung the discharge line avery minute, and two ounces return throungh the suction line. If a minimum of 30 ounces of oil in the crankcase is necessary to properly lubricate the compressor, and for some reason oil logged in the system and failed to return to the compressor, the compressor would run out of oil in 60 minutes. Under the same conditions with an oil separator having an efficiency of 80% the compressor could operate 300 minutes or 5 hours before running out of ail.

As a practical matter, there seldom are conditions in a system when no oil will be returned to the compressor, and even with low gas velocities, some fraction of the oil leaving the compressor will be returned. If there are regular intervals of full load conditions or defrost periods when oil can be returnet normally, an oil separator can help to bridge long operating periods at light load conditions. Oil separators are mandatory on systems with flooded evaporators controlled by a fload valve, on all two stage and cascade ultra-low temperature systems, and on any system where oil return is critical.

Oil separator should be considered as a system aid but not a cure-all or a substitute for good system desing. They are never 100% efficient, and in fact may have efficiencies as low as 50% depending on system operating conditions. On systems where piping design encourages oil logging in the evaporator, an oil separator can compensate for system oil return deficiencies only on a temporary basis, and may only serve to delay lubrication difficulties.

If a system is equipped with a suction accumulators, it is recommended that the oil return from the separator be connected to the suction line just ahead of the accumulator. This will provide maximum protection against returning liquid refrigerant to the crankcase. If the system is not equiped with a suction accumulator, the oil return line on suction cooled compressors may be connected to the solution line if more convenient than the crankcase, but on air cooled compressors, oil return must be made directly to the crankcase to avoid damage to the compressor valves.

If the separator is exposed to outside ambient temperatures, it must be insulated to prevent refrigerant condensation during off periods, resulting in return of liquid to the compressor crankcase. Small low wattage strap-on heaters are available for oil separators, and if any problem fron liquid condensation in the separator is anticipated, a continuously energized heater is highly recommended.

Oil Pressure Safety Control

Oil Pressure Safety Control

A major percentage of all compressor failures are caused by lack of poper lubrication. Impoper lubrication or the loss of lubrication can be dur to a shortage of oil in the system, logging of oil in the evaporator of suction line due to insufficient refrigerant migration or floodback to the compressor crankcase, failure of the oil pump, or improper operation of the refrigerant control devices.

Regardless of the initial source of the difficulty, the great majority of compressor failures due to loss of lubrication could have been prevented. Although proper system design, good preventive maintenance, and operation within the system´s desing limitations are the only cure for most of these problems, actual compressor damage usually can be averted by the use of an oil pressure safety control.

An oil pressure safety control with a time delay of 120 seconds is a mandatory requirement of the copeland warranty on all copelametic compressors having an oil pump. The control operates on the differential between oil pump pressure and crankcase pressure, and the two minute delay serves to avoid shut down during short fluctuations in oil pressure during start-up.

A trip of the oil pressure switch is a warning that the system has been without proper lubrication for a period of two minutes. Repeated trips of the oil pressure safety control are a clear indication that something in the system desing or operation requires immediate remedial action. On a well designed system, there should be no trips of the oil pressure safety control, and repeated trips should never be accepted as a normal part of the system operation.

The oil pressure safety control will not protect against all lubrication problems. It cannot detected whether the compressor is pumping oil or a combination of refrigerant and oil. If bearing truble is encountered on systems where the oil pressure safety control has not tripped, even though inspection proves it to be properly wired, with the proper pressure setting, and in good operating condition, marginal lubrication is occurring which probably is due to liquid refrigerant floodback.

Compressor Lubrication

Compressor Lubrication

An adequate supply of oil must be maintained in the crankcase at all times to insure continuous lubrication.  The normal oil level should be maintained at or slightly above the center of the sight glass while operating. An excessive amount of oil must not be allowed in the system as it may compressor valves.

Compressors leaving the copeland factory are charged with suniso 3G.  150 viscosity refrigeration oil,  and the use of any other oil must be specifically cleared with the copeland aplication Engineering Department. The naphthenic base oil has definite advantages over paraffinic base oils occurs at substantially higher temperatures with the same oil-refrigerant concentration. When this oil floats on top of the refrigerant and the oil pump inlet at the botton of the sump is fed almost pure refrigerante at start up. The resulting imprroper lubrication can result in bearing failure. Because of the lower separating temperature of suniso 3G oil, the possibility of two-phasing is greatly reduced.

Copetametic compressors are shipped with a generous supply of oil in the crankcase. However the system may require more or less oil depending on the refrigerant charge and the system desing. On field installed systems, after the system stabilizes at its normal operating conditions, it may be necessary to add or remove oil to maintain the desired level.

System Balance and Refrigerant

System Balance

If the compressor or condensing unit selected for a given application is to satisfactorily handle the refrigeration load, it must have sufficient capacity. However, over capacity can be equally as unsatisfactory as under capacity, and care must be taken to see that the compressor and evaporator balance at the desired operating conditions. Checking the proposed system operation by means of a compressor-evaporator-condenser balance chart as describet in selection 16 is recommended.

If fluctuations in the refrigeration load are to be expected, which could result in compressor operation at excessively low suction pressures, then some means of capacity control must be provided to maintain acceptable evaporating temperatures. If compressors with unloaders are not available or suitable, and if the load cannot be adequately handled by cycling the compressor. a hot gas bypass circuit may be required.



Refrigerant

Copeland compressors are primarily designed for operation with refrigerants 12, 22, and 502. Operation with other refrigerants in cascade systems may be satisfactory if the proper motor and displacement combination is selected, adequate lubrication can be maintained, and if adequate compressor protection is provided. All applications with refrigerants other than R-12, R-22 and       R-502 must be approved by the copeland aplication Engineering Departament.

R-22 is highly recommended for all single stage low temperature applications, and particularly where evaporating temperatures of -20 F. and below may be encountered. Because of the undesirable high discharge temperatures of R-22 when operated at high compression rations, R-22 should not be used in single stage low temperature compressors 5 HP and larger.

Different expansion valves are requiered for each refrigerant, so the refrigerants are not interchageable in a given system, and should never be mixed, If for some reason it is desirable to change from one refrigerant to another in an existing system by changing expansion valves and control settings providing the existing piping sizes and component working pressures are compatible. In some cases the existing motor-compressor may be satisfactory --for example, in converting from R-22 to R-502. If the conversion will result in higher power requirements as is the case in changing from R-12 to R-502. then it may also be necessary to change the motor-compressor.

The refrigerant charge should be held to the minimum required for satisfactory operation, since an abnormally high charge will create potential problems of liquid refrigerant control.

Compressor Selection

Compressor Selection

The compressor must be selected fot the capacity required at the desired operating conditions in accordance with the manufacturer´s recommendations for the refrigerant to be used. Standard copeland single stage comppresors are approved for aperation with a given refrigerant in one of following operating ranges.

                                                  Evaporating temperature
                                                 _______________________

High Temperature                   45º F. to 0º F. or 55º F. to 0º F.

Medium Temperature             25º F. to -5º F. 

Low Temperature                   0º f. to -40º F.

Extra Low Temperature         -20º F. to -40º F.


Operation at avaporating temperatures above the approved aperating range may overload the compressor motor. Operation at evaporanting temperatures below the approvet operating range is normally not a problem if the compressor motor can be adequately cooled, and discharge temperatures can be kept within allowable limits. Evaporating temperatures below -40º F. are normally beyond the practical lower limit of single stage operation because of compressor inefficiencies and excessive discharge gas temperatures. Because of problems of motor cooling or overloading, some motor-compressors may have approval for operation at limited condensing or evaporating temperatures within a given range, and if so, these limitations will be shown by limited performance curves on the specification sheet.

A give compressor may be approved in two different operating ranges with different refrigerants, for example. high temperature R-12 and low temperature R-502. sence the power requirements for a given displacement with both R-22 and R-502 are somewhat similar, in some cases a compressor may be approved in the same operating range for either of these refrigerants.

Two stage compressors may be approved for evaporating temperatures as low as -80º F., but individual compressor specifications shoould be consulted for the approved operating range. Operation at temperatures below -80º F. is normally beyond the practical efficiency range of copeland two stage compressors, and for lower evaporating temperatures, cancade systems should be employed.

Ccompressors with unloaders have individually established minimun operating evaporating temperatures since motor cooling in more critical with these compressors. As the compressor in unloaded, less refrigerant is circulated through the system, and consequently less return gas is available for motor cooling purposes.

Copeland motor-compressors should never be operated beyond published operating limits without prior approval of the copeland application Engineering Department.

Basic Application Recommendations

FUNDAMENTAL DESING PRINCIPLES

There are certain fundamental refrigeration design principles which are vital to the proper functioning of any system.

1.  The system must be clean, dry, and free from all contaminants.

2.  The compressor must be operated within safe temperature, pressure, and electrical limits.

3.  The system must be designed and operated so that poper lubrication is maintained in the compressor at all times.

4.  The system must be designed and operated so that excessive liquid refrigerantdoes not enter the compressor. Refrigeration compressors are designed to pump refrigerant vapor, and will tolerante only a limited quantity of liquid refrigerant.

5.  Poper refrigerant feed to the evaporator must be maintained, and excessive presure drop in the refrigerant piping must be avoided.

If these five steps are accomplished, then operation of the system is reasonably certain to be trouble free. If any one is neglected, Then eventual operating problems are almost certain to occur. These basic fundamentals are closely inter-related, and must always be kept in mind with regard to the aplication of any component, or whenever any change in system operation in contemplated.