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.