Page 163 - Boiler_Operators_Handbook,_Second_Edition

148 Boiler Operator’s Handbook

Compression REFRIGERANT SUPERHEAT AND SUB-COOLING
Now you might argue that Carrier system didn’t
compress the refrigerant, it pulled a vacuum. Another Just as we superheat steam by adding heat after all
look at the steam tables should reveal the volume of the the liquid has been boiled away refrigerants are super-
pound of steam at 40°F is 2423.7 ft.³ per pound of steam heated. The heat added in the compression stage by the
but at zero psig it’s 26.8. The steam Jets compressed the work done on the refrigerant to compress it increases the
steam removed from the tank at a compression ratio of refrigerant superheat. In refrigeration the temperatures
90 to 1. Now the next question may be why don’t we are simply much lower. Refrigerant in an evaporator
always use water for a refrigerant? The answer to that will continue to absorb heat by evaporating the liquid
question is that other liquids that boil at lower tem- until it runs out of liquid. In most evaporators the throt-
peratures under pressure can be used so we don’t have tling controls limit the admission of liquid such that
to create a vacuum. Absorption chillers, discussed lat- none exits in the evaporator and, because the gas tem-
er, do use water for a refrigerant. perature is still lower than the substance being cooled
With any fluid increases in pressure increase the the gas absorbs heat and that additional heat raises the
saturation temperature of the fluid. In order to elimi- temperature of the gas, superheating it.
nate the heat that was absorbed in the evaporator we Terms need to be clarified so there’s no confusion
compress the fluid to raise its saturation temperature to so I will repeat two definitions. A superheated gas is gas
a value higher than the substance the heat is rejected to. at a temperature higher than its saturation temperature
The many forms of refrigerant compressors are covered at the pressure of the gas. Superheat is the difference in
later in this chapter. temperature between the temperature of the gas and its
It takes power to compress a gas and the energy of saturation temperature. A refrigerant at 83 psig where
compression is added to the gas. Therefore heat is also the saturation temperature is 40°F may be a superheated
added to the refrigerant in the compression stage. gas at 50°F and the gas has a superheat of 10°F. The same
refrigerant compressed to 275 psig where the saturation
Condensation temperature is 120°F can be a superheated gas at 130°F
The compressed gas loses heat to a substance at and once again have 10° of superheat. Normally the su-
a temperature lower than the saturated temperature perheat after the compressor is greater than 10°.
of the gas when it condenses. The heat is transferred While superheating occurs in the evaporator,
from the refrigerant through tubing or shell into the sub-cooling occurs in the condenser. In the condenser all
substance that absorbs the rejected heat. At this point it of the gaseous refrigerant is condensed, the liquid is still
should be evident that all we are doing is transferring exposed to temperatures lower than the saturation tem-
heat, the same thing we do within our steam and wa- perature and heat leaves the liquid so its temperature
ter cycles. The construction of condensers is dependent is lower than saturation. Liquid leaving a condenser is
upon the refrigerant and the substance accepting the normally a sub-cooled liquid. The refrigerant described
rejected heat. in the previous paragraph could be condensed at 275
psig then cooled to a temperature of 110°F. In that case
Throttling sub-cooling is 10°F.
Since condensers operate at a higher pressure It’s important to understand these conditions be-
than evaporators we require a means of controlling cause lack of superheat or too much superheat can result
the flow of refrigerant from the high pressure con- in damage to compressors. Inadequate sub-cooling can
denser to the low-pressure evaporator to ensure the result in poor operation, or damage to, the throttling de-
compressed hot gases do not enter the evaporator. We vice. Inadequate sub-cooling also permits liquid flash-
also have to ensure that liquid does not leave the evap- ing to vapor before the refrigerant reaches the evapora-
orator to enter the compressor because, unlike gas, tor thus restricting the flow of the liquid refrigerant.
liquid is not very compressible. Therefore, a means Subcooling is accomplished by removing heat
of controlling refrigerant flow between the condenser from a liquid that has just condensed from a vapor to
and the evaporator is required. Many throttling sys- a temperature lower than the saturation temperature.
tems utilize means of sensing temperature to control Subcooling is normally accomplished in the condenser
the refrigerant flow while others use liquid level and but other provisions and equipment can be utilized to
two are simple orifices. subcool the liquid. A system can, for example, include
a heat exchanger that uses the cool vapor coming out