Forced Air Evaporator Coils

Forced Air Evaporator Coils

Air velocities across the coil should not exceed 500-600 FPM in order to avoid blowing water from the coil onto the load. Care should be taken to insure even air distribution across the coil, since uneven air flow can cause uneven loading of the refrigerant circuits. Fin spacing exceeding 6 per inch is not recommended because of the rapid build-up of frost on the fins. However, some users and manufacturers recommend spacing as low as 3 or 4 fins per inch, wile others report satisfactory experience with spacings as high as 8 per inch provided proper defrost controls are used.

Delivered air velocity should be adequate to insure good air circulation in the vehicle. Noise level is not a design limitation in a van, so velocities up to 10500 FPM or higher can be used.

Internal volume of the refrigerant tubes should be kept to a minimum to keep the refrigerant volume as low as possible. Since pressure drop at low temperatures is critical so far as capacit is concerned, multiple refrigerant circuits with fairly shor runs are preferred. Pressure drop in the evaporator should be no more than 1 to 2 psig. At the same time, it-is essential that velocities of refrigerant in the evaporator be high enough to avoid oil trapping. 5/8´´ evaporator tubes are acceptable, but 1/2´´ are preferred, and 3/8´´ tubing has been used succcessfully. Vertical headers should have a bottom outlet to allow gravity oil draining.

An evaporator face guard should be provided to protect the fins and tubing from cargo damage. Ample air inlet area should be provided, with access from both sides and the bottom if possible, to prevent blocking of alr to the evaporator by cargo stacked in the vehicle.

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.