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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.
