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TECHNICAL NOTES
"AN EDUCATED USER WILL HAVE BETTER CHANCE OF ORDERING THE PROPER
EQUIPMENT FOR THE JOB"
The refrigeration process requires work (electrical energy) to
pump heat from the low temperature evaporator section that
isrejected to the higher temperature ambient from the condenser
section. A mechanical refrigerant cycle requires: workingfluid
(refrigerant), compressor, condenser, thermal expansion valve (TXV)
and evaporator (chiller barrel). Additionallydepending on the
design, motor driven fans (blowers) and/or pumps will be required
to accomplish heat transfer in theevaporator and condenser
sections. Proper design requires an integrated approach to the
selection of these components.The compressor must work with the
condenser, TXV and evaporator, etc. Special constraints may require
the inclusion ofrefrigerant storage devices (accumulators and/or
receivers) and additional controls (valves) to meet
performancerequirements. Proper design of a Thermal Control System
(TCS) requires identification of the system capacity
&temperature control desired, ambient conditions (elevation
& temperature) and space constraints.
In the following paragraphs we shall briefly present the basic
issues that must be addressed in designing a TCS. Theobjective is
to present the reader with an introduction to the subject, rather
than to serve as a design manual.
System Capacity and Temperature Control
System capacity and desired temperature control must be
addressed first in designing a TCS system. Specifying the
coolingcapacity required (Btu/hr), temperature level (evaporator)
to be controlled and the allowable temperature variation isrequired
to size a TCS system. In general, component size will increase
directly with capacity and indirectly withtemperature; i.e., for a
given capacity, lowering the evaporator temperature will increase
the size of components. A myriadof control devices is available to
assist the system designer. In general, a good design should
utilize only those components
needed to satisfy specific design conditions. For example,
evaporator temperature control can be accomplished through theuse
of mechanical or electronic valves depending on the allowable
tolerance.
Ambient Conditions
Elevation and temperature will affect the performance of motors
and air breathing devices, including fans and engines. Forexample,
although the air flow rate (in cfm) is the normal parameter used in
specifying fan performance, the mass flow rateof air is actually
required to accomplish the design objective. Higher elevations,
therefore, necessitate the use of biggerfans. Also, specifying the
design capacity needed without including the ambient temperature
conditions is meaningless. Foran air-cooled condenser, coil size
will increase as the ambient temperature increases. For a given
evaporator temperature,the compressor size will similarly increase
directly \with ambient (head) temperature.
Space Constraints
The physical space available can have a direct impact on TCS
design and cost. Where space is not a primary concern,components
can be selected primarily based on costs. In general, space
constraints can be satisfied; e.g. a smaller (facearea) condenser
coil can be used, providing a larger condenser fan (>cfm) is
employed. Each of these design choices is atradeoff of various
parameters; increasing the air flow rate (cfm) of a fan will
increase the performance of a coil, at theexpense of additional
noise.
Section 1: Compressor Selection
The compressor is the "heart" of the refrigeration system. It
mechanically compresses the low temperature, low-pressuresuction
gas into high temperature, high pressure superheated gas. Saturated
conditions exist when a gas and liquid areallowed to stabilize
within a confined space at given ambient conditions. At each
temperature, an equilibrium condition willbe established between
the pressure and temperature. Under saturated conditions,
specifying either temperature orpressure, will establish the
remaining condition for a known refrigerant. For example,
specifying that the refrigerant is R22 ata saturated temperature of
70 degrees F, equates to a working pressure of approximately 122
psig. The correlation betweensaturated pressures and temperatures
is presented in various sources (such as ASHRAE Fundamentals). A
superheated
vapor exists when the pressure at a given temperature is raised
above saturated conditions. This occurs at the inlet of
thecompressor. The refrigerant will be evaporated to a superheated
state to assure that liquid is not introduced into thecompressor,
as liquid slugging is a common cause of compressor failures. The
compressor elevates the low-pressuresuction gas, into high pressure
superheated vapor. A refrigerant compressor is a pump designed to
work with a gaseousmedia. Depending on the application, compressor
designs may be either reciprocating or rotary and of hermetic,
semihermetic or open design. Reciprocating compressors contain
cylinders and use pistons to compress the refrigerant gas.Rotary
compressors use an "eccentric" cam type action to compress the
refrigerant and push it from the low-pressure sideof the cycle to
the high-pressure side. Hermetic compressors contain both the
electric motor and the mechanicalcomponents within a single sealed
housing. Repairs are normally only accomplished at a factory
equipped to open the shelland replace worn or damaged parts. The
refrigerant flow through the compressor is used to cool the
electric motor. Semi-hermetic compressors also contain both the
electric motor and mechanical elements within a single housing, but
permitaccess to major components, permitting minor field repairs.
Major re-builds are generally accomplished off site at a
facility
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set up for this operation. Open drive compressors separate the
prime driver (electric motor, gas engine, etc.) from thecompressor.
A shaft connects the mechanical compressor components to the prime
driver, which may be coupled directly orvia belts. Semi hermetic
and hermetic compressors contain an integral motor, while the open
drive compressor simplycontains the compressor mechanical
components (e.g., cylinders, pistons and housing) and requires a
shaft to connect tothe prime driver. The shaft contains a seal that
must be maintained in a "wet" state in order to avoid refrigerant
leaks. It isnormally required that an open drive system not be idle
for an extended period of time to avoid drying out the seal
andcausing refrigerant leaks.
2. Condenser
The hot, superheated gas exiting the compressor is condensed
into a high-pressure liquid in the condenser section. This
isaccomplished either by an air cooled condenser finned tube coil,
or a shell and tube barrel. In either, design, the coolantmedia
(air, water or other fluid) extracts heat from the refrigerant,
causing it to condense from a high pressure, hightemperature
superheated vapor into a high pressure, high temperature sub cooled
liquid. The condenser rejects theevaporator load, the compressor
heat of rejection and the condenser fan (motor load) in the case of
a pusher system. In anair-cooled design, the ambient air is pushed
(or pulled) through the coil. This is accomplished either via a
propeller type fanor blower, depending on the flow rate and
pressure drop; higher flow rates (cfm) and larger pressure drops
require blowersrather than fans. Convective heat transfer and the
associated laws drive the heat transfer from the hot refrigerant
gas to thehigh temperature ambient. In a liquid cooled design, a
pump is required to pump the cooling media (such as water)
throughthe condenser heat exchanger. The system heat may ultimately
be rejected via a cooling tower or chiller system. Coil sizesare
directly effected by allowable airflow rates (cfm) and system
pressure drops. Smaller face area coils are possible withlarger
flow rates (cfm), at the expense of larger pressure drops
(requiring bigger fans or larger motors). Proper coil
selectionrequires balancing the allowable face area of the coil
(size) with the flow rate (cfm) and pressure drop allowable to
achieve
the desired performance.
3. Thermo Expansion Valve (TXV)
The high temperature, high-pressure liquid must be expanded into
a low pressure, low temperature fluid to perform therefrigeration
effect. A thermal expansion device may be accomplished via a simple
capillary type tube, or a TXV. Expansionof the liquid leaving the
compressor can be accomplished simply by passing the refrigerant
through a tube of restricteddiameter (capillary tube). The added
pressure drop causes the high temperature, high-pressure vapor to
"flash to a lowtemperature low pressure fluid. The liquid/vapor
refrigerant leaving the expansion device is a mixture, primarily
composed ofliquid. A capillary tube system is primarily used on
smaller systems, where the refrigerant load is fairly constant. For
bettercontrol and on larger systems, a TXV valve is normally used.
The valve may be designed to sense both the pressure andtemperature
of the gas leaving the evaporator to assure that cooling is
accomplished and that the compressor sees purevapor.
4. Evaporator
The refrigerant mixture is transformed into a superheated vapor
in the evaporator. This may be accomplished either byan air-cooled
finned tube coil, or a shell and tube barrel. In either, design,
the coolant media (air, water or other fluid)the refrigerant
extracts heat from the air, causing it to evaporate from a
primarily liquid state into a superheated vapor.The evaporator
absorbs the refrigerant load and in the case of a pusher type
evaporator blower, the motor load. In anair-cooled design, the
ambient air is pushed (or pulled) through the coil. This is usually
accomplished via a blower, toprovide adequate head to overcome the
coil resistance and the system pressure drop. Convective heat
transfer and theassociated laws drive the heat transfer from the
hot refrigerant gas to the high temperature ambient. In a liquid
cooleddesign, a pump is required to pump the cooling media (such as
water) through the evaporator heat exchanger (shell &tube, or
chiller barrel). In an ideal system, the refrigerant would be
transformed from a liquid vapor mixture into asaturated vapor at
the exit of the evaporator. However, in order to assure complete
evaporation and primarily to protectthe compressor, the refrigerant
is transformed into a superheated vapor at the exit of the
evaporator. Coil sizes aredirectly affected by allowable airflow
rates (cfm) and system pressure drops. Smaller face area coils are
possible withlarger flow rates (cfm), at the expense of larger
pressure drops (requiring bigger fans or larger motors). Proper
coilselection requires balancing the allowable face area of the
coil (size) with the flow rate (cfm) and pressure drop
allowable to achieve the desired performance.
