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1 2010 Emerson Climate Technologies

Printed in the U.S.A.

AE17-1251 R3

Application Engineering

B U L L E T I N

A compressor's ability to survive under adverse ambientconditions varies with both compressor and systemdesign. The system designer must realize that a unitthat might operate quite satisfactorily with a low failurerate under mild ambient conditions may be completelyunsuitable for operation in an extreme environment,whether it might be in northern Canada or the Arabiandesert. The heat pump failure fiasco in the United Statesin the late 1950's and early 1960's should serve as achilling reminder of what can happen when a productis thrown into a market place without sufficient regardfor its operating limitations.

With the increasing importance of the Mid-Eastand equatorial Africa in world air conditioning andrefrigeration markets, any company exporting ordesigning air-cooled equipment for those areas shouldreview their design standards to be sure they areadequate. In areas close to the equator, radiation fromthe sun's rays and reflection from a roof top or concretecan result in effective air temperatures entering anair-cooled condenser considerably in excess of themeasured air temperature. Knowledgeable engineerswith experience in the Mid-East feel that a unit to besafely applied without restriction in that area mustbe capable of continuous operation with entering airtemperatures of 125F.

Obviously the most critical area of unit design for highambients is the condenser. The higher the condensingpressure for a given evaporator pressure, the higher thecompression ratio and the more critical the dischargetemperature. In general, for high ambient areas,Emerson Climate Technologies would recommend anair to condensing temperature difference of 10F for lowtemperature units, 15 to 20F for medium temperature,and 20 to 25F for high temperature. A direct air blaston the compressor will help in maintaining acceptable

oil temperatures, and minimum superheat in the suctionvapor entering the compressor is advisable. In criticalapplications it may be necessary to insulate the suctionline.

Recent field investigation in the Middle East indicatestwo specific areas where system design may beimproved to obtain better reliability.

Single phase motor failures appear to be closely relatedto starting difficulty under low voltage and high pressure

differentials. It is recommended that supplementary startdevices be applied with all single phase compressors.The solid state PTC devices now on the market offer areasonable increase In starting torque with no penaltyin electrical reliability.

On through-the-wall window air conditioners, units withside air inlets are extremely vulnerable to thick walls andsloppy installations. It would be our recommendationthat all through-the-wall units intended for the Mid-Eastmarket be designed with both air inlet and dischargeon the face of the unit to achieve better air flow overthe condenser.

Users should not be expected to be compressorexperts, but certainly system designers should havethe engineering expertise and sophistication to evaluatealternate compressor designs with regard to durabilityand reliability. There are fundamental differences indesign between a welded as opposed to an accessiblehermetic compressor, differences which are reflectedin both cost and performance. From an operatingstandpoint, the most significant difference is the ability ofthe compressor to reject heat to the surrounding air. Anaccessible hermetic compressor has the motor in directphysical contact with the compressor body, so heatconductivity to the external surface is highly efficient. Ofeven more importance, the compressor head where gasis discharged from the cylinder at its highest temperatureis directly exposed to the ambient air. In contrast, on awelded compressor both the motor and the compressorhead are totally surrounded by refrigerant vapor, there isvery little direct metal to metal conduction to the outsideshell, and the greatest proportion of the heat rejectedgoes directly into the refrigerant vapor.

In high ambient areas where effective heat rejection iscritical for compressor survival, conservative system

design may be critical in the survival of any compressor,and certainly is far more critical with welded compressordesign than with accessible-hermetic.

Users frequently ask for critical temperature limits incompressor application, with the optimistic hope thatthere is a magic black and white guideline that willinsure satisfactory operation. As with most things in lifethere is no easy answer, since the time a compressormust operate at extreme stress conditions is of equalimportance.

Design Considerations for High Ambient Conditions

AE17-1251 R3 September 1978Reformatted October 2010

Application Engineering

B U L L E T I N

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2 2010 Emerson Climate Technologies

Printed in the U.S.A.

AE17-1251 R3

Application Engineering

B U L L E T I N

It might be helpful to visualize a compressor's lifeexpectancy in terms of concentric circles, much likethe life rings on a tree trunk. The center would be the

most optimum operating conditions, normally the midpoint of the compressor's design operating range.The outermost circle would represent the limit of thecompressor's operation, which might be determinedby marginal temperature, voltage, motor loading, orlubrication. Assume the compressor's life expectancyat the center to be 25 years, and at the limits to be 25days. Operation at those limits for many hours might notbe harmful if the majority of the operating hours werewell within the limits of easier operating conditions. Butunder continuous or long term operating conditions, thecompressor's life expectancy is going to be decreasedas the operating limits are approached, and the designengineer who assumes the same life expectancy at the

limits of compressor operation that might be expectedin the center of the operating range is being unrealistic.

The most critical temperature conditions are probablynot at maximum load, but at the higher compressionratios created by high condensing temperatures andlow evaporating temperatures. For high ambientapplications, operating tests should be run to evaluatethe performance of the system at normal evaporatingtemperatures and high condensing temperatures.

Most refrigeration oils will start to break down orcarbonize at temperatures of 350F. The rate anddegree of chemical reaction are undoubtedly related

to the amount of oxygen and moisture in the system.Since the practical environment existing in an operatingcompressor of larger horsepower size is seldom

contaminant free, field vulnerability may be much greaterthan in a carefully controlled laboratory experiment.The hottest temperature exists at the discharge valve

port, and normally the discharge line within 6 inches ofthe compressor outlet will be from 50F to 75F coolerthan the discharge port. Therefore 275F dischargeline temperatures represent an extreme temperaturecondition, 250F is usually considered a danger level,and 225F and below is desirable for reasonable lifeexpectancy.

There are two schools of thought in the industry onoil sump temperatures. The more conservative viewis that oil temperatures of 200F or higher result ingreatly decreased viscosity and marginal lubrication.On welded compressors, oil temperatures up to 240Fare considered acceptable on some models. Obviously

the characteristics of the lubricant are critical, andbearings must be capable of withstanding the extremetemperatures. Lower temperatures are generally muchmore conducive to long life.

In the booming economy of the Mid-East in particular,where high ambient temperatures are a way of life,where technical expertise and service support aregoing to be stretched to the limit for years to come,reliability under adverse conditions may well become afar more important competitive factor than first cost. Thecost of repetitive field failures could very well becomeprohibitive.

Obviously the suitability of a unit design becomes amatter of engineering judgment, and good judgmentmay be vital to a successful product.