7/28/2019 AE1234

1/2

1 2010 Emerson Climate Technologies

Printed in the U.S.A.

AE17-1234 R4

Application Engineering

B U L L E T I N

Any refrigeration or air conditioning system utilizing anair-cooled condenser which must operate during spring,fall or winter months must be designed to properly controlliquid refrigerant fed to the evaporator under widelyvarying ambient conditions. Since the flow characteristicsof capillary tubes and expansion valves are proportionalto the differential pressure across the capillary tube orvalve, the problem is most critical under low outdoorambient conditions when the condensing pressure canfall to very low levels without some means of control.

With a conventional restrictive device, excessively low

condensing pressure will reduce the refrigerant flowrate to a point where evaporating pressures can fall toa dangerously low level, and can result in evaporatorfrosting, compressor short cycling, liquid refrigerantfloodback and other malfunctions; all of which can leadto compressor failure.

In order to maintain satisfactory operation under lowambient conditions, a number of approaches are possible,each of which has both advantages and disadvantages.Normally some type of outdoor precautions must betaken if compressor operation is anticipated on air-cooledsystems in ambients of 50 F or below.

1. Maintaining Constant Air Temperature toCondenser.

An approach that has proven to be highly satisfactoryon supermarket installations and other applicationswhere large numbers of compressors are involved isto locate the entire condensing unit inside a machineroom. The temperature in the machine room thencan be controlled within reasonable limits by meansof a thermostat controlling automatic air inlet louversand exhaust fans. In effect, the condensing unitsees a continuous stable operating environment.

Obviously this involves space and expense, andis used only when sufficient units are involved toprovide a continuous source of heat.

2. Reducing condenser Capacity by FloodingWith Liquid Refrigerant

A number of manufacturers offer valves to maintainhead pressure by partially flooding the condenserwith liquid refrigerant. Various combinations ofvalves may act to restrict or stop refrigerant flowthrough the condenser while at the same timebypassing discharge gas directly to the receiver

as necessary to maintain a preset condensingpressure. Under normal ambient conditions, thebypass valve is closed and refrigerant flows freelythrough the condenser.

Under operating conditions, theflooding type controlprovides good modulation and stable head pressureconditions. However systems of this type do requirea considerable quantity of extra refrigerant in order toflood the condenser, possibly requiring an oversizereceiver. If insufficient refrigerant is available tofloodthe condenser, the hot gas bypass valve may feed

continuously, and this can create major problemswith expansion valve control. If excessive refrigerantis added to the system, it may be impossible topump the system down and isolate the refrigerantin the receiver and condenser.

Due to the large refrigerant charge in the systema pumpdown control is highly advisable for propercompressor protection against liquid refrigerantmigration and slugging on start up.

3. Reducing Condenser Capacity by ReducingCondenser Air Flow

Another means of maintaining adequate condensing

pressure which has the advantage of not floodingthe condenser is provided by limiting the airflowover the condenser surface.

This can be accomplished most simply andeconomically by cycling the condenser fan froma reverse acting high pressure control, but theresulting rapidfluctuation in head pressure can resultin erratic liquid refrigerant control. Where variationsin ambient temperature are not too severe, this mayprovide adequate control, but for larger systemsand particularly where air temperatures entering thecondenser of 0F and below may be encountered,better modulation of control is required to prevent

potential compressor problems.

Suction-cooled compressors operating withevaporating temperature below 0F and air-cooledcompressors at any evaporator temperature,require constant airflow across the compressor foradequate cooling. Fan cycling on these applicationsis not acceptable unless auxiliary cooling ofthe compressor is provided. Cylinder and headtemperatures are most likely to be extreme athigh compression ratios, and the probability is that

Low Ambient Compressor Operation

AE17-1234 R4 March 1993Reformatted October 2010

Application Engineering

B U L L E T I N

7/28/2019 AE1234

2/2

2 2010 Emerson Climate Technologies

Printed in the U.S.A.

AE17-1234 R4

Application Engineering

B U L L E T I N

reduced loads and increased compressor capacityare most likely to result in high compression ratiosduring low ambient conditions.

Where condensers with multiple fans are used,cycling of individual fans on a staggered basisgives more stable pressure control, but problemsstill exist if conditions are such that the last fan cancycle on and off.

Better modulation can be obtained by varying thefan motor speed, and this is now possible withavailable solid state controls and suitable motors.

However even if the condenser fan is completelystopped, the condenser must be protected from thewind to avoid loss of control on cold windy days,and a suitable shroud must be provided.

Another very effective means of reducing airflowover the condenser is by means of power operatedshutters actuated by the refrigerant pressure.

Even though extra refrigerant is not required forsystems utilizing fan control or shutters, pumpdowncontrol is still highly recommended for any unit inwhich the compressor and suction line are exposedto cold ambient conditions, where the compressormust start and operate under such conditions.

4. Starting At Low Ambient Temperatures

At times, the temperature to which the condenser

and receiver may be exposed is so low that thereis insufficient pressure available to start the feed ofliquid refrigerant to the evaporator on start up. As aresult the compressor may pull a deep vacuum onthe system within a matter of seconds, and shortcycle on the low pressure control. If the low pressurecut-out is set low enough to prevent short cycling,then the setting may be so low that it no longerprovides adequate protection for the compressor,and cannot be used for pumpdown control.

It is possible to use a low pressure control with atime delay. This may provide sufficient time to buildup head pressure so the unit can then operate

normally, but if the same time delay is effectiveduring pumpdown, this can allow the compressor topull an undesirable deep vacuum prior to shutdown.

One proprietary system provides quick receiverpressure by diverting the compressor dischargegas directly to the receiver on start-up, bypassingthe condenser until a preset pressure is obtained.This provides excellent start-up pressure under allweather conditions.

Another solution to the start-up problem is the useof a small electric heater band or element on aninsulated receiver to maintain a temperature suchthat the saturated pressure will be adequate toinsure refrigerant flow to the evaporator on start-up. The heater can be controlled by a thermostat,while backflow to the condenser can be preventedby a check valve. This type of start-up control, inconjunction with a condensing pressure control,deserves more widespread consideration and usagethan it has received.

5. Operating With Low Head Pressure

Some proprietary systems are utilizing expansionvalves designed to operate at very low differentialpressures in order to take advantage of the increasedcapacity and reduced power consumption resulting

from compressor operation at low condensingpressures. Since a valve with less restriction willalso be less sensitive in its control characteristics,additional system protective features such assuction accumulators and heat exchangers maybe necessary to prevent excessively wet refrigerantvapor from reaching the compressor. A minimumof 15 F to 20 F superheat at the compressor isessential to insure proper lubrication.

6. Use Of Crankcase Heaters

Crankcase heaters are strongly recommended onall refrigeration systems where the compressoris exposed to cold ambient temperatures, on all

split air conditioning systems, and on package airconditioning equipment 5 H.P. and larger. As therefrigerant charge increases, the start-up problemsassociated with liquid refrigerant become morecritical.

The crankcase heater will minimize liquid migration tothe crankcase during periods when the compressoris not operating. Another important function of thecrankcase heater on outdoor units is to keep thecompressor oil warm enough so it remains fluid.On rooftop units exposed to winter winds, the oilmay become so viscous without a crankcase heaterthat the compressor cannot develop oil pressure.

In summary, a number of different approaches forproper cold weather control may be acceptableif properly applied. Such factors as system size,type of application, severity of weather conditionsto be expected and unit location must be taken intoconsideration. The design goal must be to maintainoperating conditions under the most adverseweather conditions which will allow the compressorto operate safely.