Isentropic compression means compression at constant entropy. The entropy of gas leaving the compressor will be identical to that of gas entering the compressor. Pressure of gas as it leaves the compressor is determined by the vapor pressure of liquid refrigerant in the condenser. Assuming, that there is an open pipe between the condenser and outlet valve on the compressor cylinder and ignoring the small pressure drop that will be present, pressure in the discharge line will be constant all the way from condenser to compressor. Accordingly, in passing through the compressor, there has been an increase in temperature and heat content or enthalpy.

CAPACITY

Previously all references to changes in heat content or enthalpy have been on the basis of one pound of refrigerant. In order to relate to capacity, the element of time must be introduced. The capacity of the system depends on how often a pound of refrigerant evaporates and this depends on compressor size and on properties of the refrigerant at refrigerant cycle temperatures.

In an example with R-12, assume the compressor is of the right size to deliver one ton of refrigeration when refrigerant evaporates at 0 deg. F and is condensed at 100 deg. F. as each pound of refrigerant R-12 can absorb 49.06 Btu. Accordingly this amount of heat is available for cooling work. Since one ton of refrigeration is equal to 200 Btu/min, one can produce this amount of refrigeration by using this formula:

Refrigeration circulated = (200 Btu/min) / (49.06 Btu/lb) = 4.077 lb/min.

HEAT OF COMPRESSION

Heat added to refrigerant gas by compressing it has been found to be 15.804 Btu/lb. Knowing the rate at which the refrigerant circulates, calculation of the amount of heat added on a time basis is as follows:

15,804 Btu/lb x 4.077 lb/min = 64.433 Btu/min.

POWER OF COMPRESSION

Energy can be expressed in different ways but the unit of primary interest in refrigeration is horsepower. By definition one horsepower = 42.42 Btu/min., so power required to compress the gas can be calculated.

Horsepower of compression = 64.43 Btu/min. / 42.42 Btu/min. = 1.519 HP/ton of refrigeration

Theoretically about one and half horsepower is required to produce on ton of refrigeration with R-12 at an evaporating temperature o 0 deg F and condensing temperature 100 deg F. actual horsepower required will be greater depending on volumetric and mechanical efficiency of compressor and also whether or not entropy of the gas remains constant during compression.

COMPRESSOR DISPLACEMENT

An idea of the size of the compressor needed for refrigeration application can be obtained from property of the refrigerant. In the R-12 example at 0 deg. F. evaporating and 100 deg. F. condensing temperatures, net refrigeration effect is 49.06 Btu/lb. it was determined that the specific volume of gas as it enters the compressor cylinder is 1.94 cu ft/lb. Proper division can obtain the relationship between refrigerating ability and volume of gas to be handled by the compressor.

49.06 Btu/lb / 1.94 cu ft/lb = 25.3 Btu/cu ft

The above relationship does not indicate time required to remove the heat. To produce cooling at the rate of 200 Btu/min (1 ton) the corresponding displacement can be calculated as follows:

Compressor displacement = 200 Btu/min / 25.2 Btu/cu ft = 7.91 cu ft/min.

COMPRESSOR RATIO

The temperature of boiling refrigerant in the evaporator establishes inlet pressure to the compressor. Outlet or discharge pressure from the compressor is established by the temperature of condensing refrigerant in the condenser. The ratio of these two pressures is call compression ratio and indicates levels of pressure between which the compressor must be able to operate. In determining compression ratio absolute pressure must always be used. For example, the vapor pressure of R-12 liquid at 100 deg. F is 131.86 Pisa and vapor pressure at 0 deg. F is 23.849 Pisa compressor ratio, then, is:

131.86 Pisa / 23.849 Pisa = 5.5

A low compression ratio is desirable in the first place, because low ratio means low power consumption in compressing the gas. This is true regardless of the level of inlet pressure. For example, it would take about the same increase in Btu/lb to compress from 100 Pisa to 500 Pisa as it would to compress from 10 Pisa to 50 Pisa. This change in heat content per pound of refrigerant, that is, number of pounds per minute or refrigerant passing through the compressor. Flow rate would be different at different levels of refrigerant inlet pressure. A low compression ratio is also desirable because of its effect on volumetric efficiency of compressor as explained in the next paragraph.

VOLUMETRIC EFFICIENCY

The piston in a compressor cylinder cannot go all the way to the top of the cylinder on its compression stroke since a little room must be left for the action of the valve. Any contact between the piston and the top of the cylinder would be very undesirable. Space at the top of the cylinder when the piston is as high as it can go is called clearance volume. Refrigerant gas in this volume is not expelled from the cylinder and remains behind when the piston goes through its downward stroke. Power required to compress this amount of gas does not do any useful work and represents a loss factor in compressor operation. Wasted power may vary from 10% to 50% of total power applied to the compressor depending on its mechanical construction, nature of refrigerant and operating conditions of the system. Ratio of usable power to total power is called volumetric efficiency and is usually determined experimentally for each compressor design.

Volumetric efficiency is directly affected by compression ratio. At high compression ratios, pressure of refrigerant gas in the clearance volume is high with respect to inlet or suction pressure. As a result, a higher percentage of total power is wasted in compressing this portion of refrigerant gas and volumetric efficiency is lower.

EFFECT OF SUPERHEAT

As previously mentioned superheat in return suction gas lines may have some disadvantages. In order to illustrate this possibility, let us assume that R-12, while still evaporating at 0 deg. F, has absorbed enough heat to raise the temperature to 120 deg. F as it enters the compressor cylinder. Refrigerant gas at this point then will have the following properties.

• Temperature        = 120 deg. F

• Specific volume    =    2.103 cu ft/lb

• Heat content        =    95.062 Btu/lb

• Entropy               = 0.2032 Btu/(lb) (deg. F)

Assuming that isentropic compression occurs and that entropy stays the same, properties of the gas as it leaves the compressor, at a condensing temperature of 100 deg. F, will be:

Temperature = 243 deg. F

Heat content = 112.181 Btu/lb

By comparison these properties with those shown above for a superheat temperature of 80 deg. F, for gas for gas entering the compressor cylinder, some difference can be found. Volume of each pound of refrigerant at the higher temperature is 2.10 cu ft instead of the 1.94 cu ft at the lower temperature. Since the compressor speed cannot be changed (at least in a hermetic compressor), its displacement in terms of cu ft/min. cannot be changed. Accordingly, the compressor will have to run longer to handle each pound of refrigerant. This is reflected in a lesser amount of refrigerant circulated in lbs/min. On the other hand, as long as the condensing temperature, evaporating temperature, and amount of superheat added to refrigerant inside of the evaporator all remain the same, the net refrigerating effect of each pound of refrigerant also is unchanged. Since fewer pounds of refrigerant are now able to be handled by the compressor, refrigerating capacity of the system decreases. In the example used, capacity changes from 200 Btu/min. with 80 deg. F superheat temperature to 185 Btu/min. if the temperature rises to 120 deg. F, or a decrease of 7.5%.

EFFECT OF CONDENSING TEMPERATURE

As a further illustration of the effect of changing conditions on refrigeration system capacity, assume in the R-12 example that the condensing temperature is 120 deg. F instead of 100 deg. F and the evaporating temperature remains 0 deg. F, the net refrigerating effect will then be:

Heat content of gas with 20 deg. F of superheat = 80.161 Btu/lb

Heat content of liquid at 120 deg. F = 36.013 Btu/lb

Net refrigerating effect = 44.148 Btu/lb

It will be noted that the net refrigerating effect is lower when the condensing temperature is 120 deg. F than when it is 100 deg. F. Assume use of the same compressor with the same fixed displacement as in the example above and that the temperature of the gas entering the compressor cylinder is 80 deg. F and has a specific volume of 1.94 cu ft/lb., the weight of refrigerant that the compressor is able to circulate will still remain the same.

7.91 cu ft/min. / 1.94 cu ft/min = 4.08 lb/min.

Since each pound of refrigerant is able to do less cooling, total capacity of the system will be less.

44.15 Btu/lb x 4.08 lb/min = 180 Btu/min

With the condenser at 100 deg. F, refrigerating capacity was 200 Btu/min. So if the condensing temperature is raised to 120 deg. F, there will be about a 10% reduction in capacity if all other conditions remain the same.

CHANGES IN OPERATING CONDITIONS

By similar calculations of refrigerant and refrigerating properties it will be determined that for a given compressor, refrigeration capacity is increased by the following changes in operating conditions. In this summary, it is assumed that the condition listed is changed without affecting other conditions or properties of refrigerant. In practice, this is not always possible and, in some cases, a change in one condition is accompanied by an opposing change in another condition and both changes must be taken into consideration in determining change in capacity.

INCREASED CAPACITY:

1. Lower condensing temperature (or lowering the liquid temperature by any means, such as using a liquid vapor heat exchanger)

2. Higher evaporating temperature

3. Higher superheat in the evaporator

4. Lower temperature of gas entering the compressor cylinder

THE CONDENSER

The last step in the refrigeration cycle is to dispose of the heat that the refrigerant gas has picked up in the evaporator. In going through the compressor, the function of the condenser is to remove heat from the refrigerant vapor and transfer it to something else, usually air or water.

When refrigerant vapor leaves the compressor, it is in a superheated state, that is, it contains more heat than if it were saturated vapor at the same pressure. Pressure at which the compressor discharges vapor from the cylinder is regulated by the temperature of the liquid in the condenser. Whenever liquid is present in a piping system, unrestricted by orifices or small passageways, pressure through system will be vapor pressure of liquid at its temperature except for very small differences due to pressure drop as the vapor passes through the piping. Discharge pressure can be found by referring to tables of properties for saturated liquid at the condensing temperature. Using R-12 as an example and assuming that the condensing temperature is 100 deg. F, discharge pressure will be 117.2 psig.

As more heat is removed from saturated vapor of refrigerant in the condenser, it changes to a liquid without any change in temperature. Latent heat of vaporization or, in this case, condensation, since the same amount of heat is involved in going either from a liquid to a vapor or from a vapor to a liquid, at 100 deg. F it is 55.93 Btu/lb. when this amount of heat has been removed, a pound of refrigerant vapor has been completely changed into liquid.

The temperature of material used in cooling the condenser, must be at a lower temperature than the refrigerant being cooled and condensed. Or, explained in another way, if a cooling medium is available in the condenser, at a given temperature and flow rate, the condensing temperature of the refrigerant will automatically be established at some temperature higher than that of the cooling material. When air is used as the cooling fluid, the refrigerant condensing temperature may usually be in the order of 20 to 30 deg. F higher than the air temperature. With water cooled condensers, the condensing temperature would more likely be 8 to 10 deg. F higher than the water temperature.

The condensed liquid refrigerant leaves the condenser and is carried back toward the evaporator ready to begin another cycle.

REDUCING HIGH PRESSURE LIQUID TO LOW PRESSURE LIQUID

The liquid refrigerant in the liquid line is at a high pressure (condensing pressure of 117 psig). After it gets into the evaporator, its pressure must be down to 37 psig in order that it will boil at 40 deg. F; some means must be provided at the inlet of the evaporator to reduce the 117 psig to 37 psig. There are several means of reducing the liquid from condensing pressure to evaporator pressure, but they all depend upon a small orifice in the valve known as an expansion valve, float valve or injector, or a length of tubing of small inside diameter, known as capillary tube or restrictor tube. These restrict the flow of refrigerant and cause a pressure drop from condensing pressure to evaporator pressure.

The real process of producing the low temperature is done in the evaporator; the only function of the compressor and condenser is to salvage the vapor from the evaporator by reconverting it to a liquid so that it can again be used in the evaporator.

NET REFRIGERATING EFFECT

The expansion valve is adjusted to reduce the pressure from 117 psig to 37 psig, so the pressure in the evaporator is 37 psig and the temperature 40 deg. F, for at 37 psig, R-12 boils at 40 deg. F.

Heat is required, however, so the boiling refrigerant absorbs heat from the evaporator and from the liquid around the evaporator. We can even determine how much heat each pound of refrigerant that is boiled absorbs; that is, each pound that is circulated through the evaporator and the rest of the system.

The amount of heat each pound of refrigerant picks up from the product or air as it travels through the evaporator is called the refrigerating effect. The liquid refrigerant entering the metering device (TX valve) and evaporator coil has certain heat content at its given temperature, as does the refrigerant vapor leaving the evaporator at its lower temperature. The difference in heat content of these two stages is the amount of heat absorbed by the pound of refrigerant as it circulates through the evaporator. Therefore, the refrigerating effect is rated in terms of Btu per pound of refrigerant circulated.

The heat absorbed by the refrigerant depends on two main conditions of the refrigerant and the temperatures at these conditions:

1. the temperature of the liquid refrigerant entering the refrigerant control (TX valve)

2. the evaporating temperature, or the temperature of refrigerant vapor leaving the evaporator.

Table which lists the properties of R-12 in a saturated vapor state, list the enthalpy (heat content) of the liquid refrigerant at 100 deg. F at 31.16 Btu/lb; whereas the enthalpy of 40 deg. F refrigerant vapor leaving the evaporator at 82.71 Btu/lb. Therefore the difference between these two figures amounts to 51.55 Btu/lb, or the amount of heat that each pound of refrigerant absorbs from the product or air under the given conditions.

As already pointed out, the two variables that will alter the refrigerating effect per pound of refrigerant circulated involve the "entering refrigerant liquid temperature" and "leaving refrigerant vapor temperature." Therefore, by lowering the entering liquid temperature, the refrigerating effect will be increased. This means that fewer pounds of refrigerant will have to be circulated to do the work required. Also, by raising the evaporating temperature while the condensing temperature remains the same, the amount of refrigerant needed to be circulated will be lowered.

DIRECT-EXPANSION EVAPORATOR COIL CAPACITY

The capacity of any direct-expansion (DX) evaporator or cooling coil is dependant upon:

1. the temperature of the refrigerant circulated

2. the temperature of the air (dry and wet bulb) or other liquid being circulated through the evaporator or the cooling coil

3. the volume of the cooled liquid being circulated

If the temperature of the cooled liquid entering the evaporator is varied, the refrigerating effect is also varied. This affects the capacity of the evaporator, and, if the temperature of the cooled liquid remains the same, any variation in the refrigerant suction temperature will also change the temperature difference between the refrigerant and the liquid being cooled. If this temperature difference decreases, the rate at which the refrigerant evaporates also will decrease.

The same decrease in refrigerant evaporation will occur if the quantity of liquid being cooled is decreased, since the lesser amount of liquid will cool to a lower temperature and reduce the temperature difference between the refrigerant and the liquid being cooled.

In the direct-expansion evaporator capacity tables it shows that, as the suction temperature is increased the coil capacity is lowered. This is caused by the decrease in the temperature difference between the refrigerant and the liquid being cooled.

The identical, conditions affects the evaporator and the compressor differently: as the capacity of the evaporator increases, the capacity of the compressor decreases. Therefore, components that are selected and installed in the field require careful balancing of the evaporator and the compressor components (with regards to their individual capacities) in order to find out the point or points at which each will have the same capacity.

OIL CIRCULATION

Oil travels in the refrigeration cycle, along with the refrigerant, for the lubrication of the moving parts since the compressor must be lubricated, the flow of oil will be considered from that initial point. The refrigerant vapor comes in direct contact with oil clinging to the cylinder walls as the pistons are lubricated. Some of the oil is carried along with the refrigerant vapor as it passes into the hot gas discharge line between the compressor and the condenser.

If only a small amount of oil travels along with the refrigerant, it will pass into the condenser and receiver and through the liquid line into the evaporator, returning to the compressor crankcase before that segment of the system is pumped short of oil. But some compressors may pump a relatively large volume of oil, and unless provisions are made for its speedy return to the crankcase, serious damage can occur to the compressor. A precautionary measure is the correct installation of refrigeration lines having the proper pitch to correctly-sized lines, with oil loops provided in the piping system where needed. In some cases installation of an oil separator in the refrigeration circuit close to the compressor discharge may be required. Small amounts of refrigerating oil will not be harmful to the evaporator, but large amounts collecting in the circuits or passages of the evaporator will cause an increase in evaporator temperature. There will be less total cooling if the suction pressure remains constant, so the entire system will be less efficient.

If the oil is allowed to remain in the evaporator, it will take up space in the coils that should be used for the vaporization of the refrigerant, and refrigeration will decrease in efficiency.

Disclaimer - While Berg Chilling Systems Inc. ("Berg") makes reasonable efforts in providing accurate information, we make no representations or warranties regarding the accuracy of any content therein. We assume no liability or responsibility for any typographical, content or other errors or omissions. We reserve the right to modify the content of this documentation without advance notice.