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6
REFRIGERATION CYCLE OF FREON 12
 
A. GENERAL THEORY
 
6A1. Main elements in the cycle. In Section 4A1, a brief statement of the principles of mechanical refrigeration was given. In this section, a detailed explanation of the full cycle of operations is presented.

In the circuit of mechanisms through which the refrigerant Freon 12 flows, there are five main elements. Starting from the point where we wish to remove heat, they are: 1) evaporator, 2) compressor, 3) condenser, 4) liquid receiver, and 5) expansion valve. In addition, various control and safety devices are connected into the circuit.

6A2. The heat pump. The refrigerant cycle is more easily understood when it is compared to the flow of water. Everyone knows that

  water naturally flows downhill that is, it always flows from a higher level to a lower level under the pull of gravity. If it is necessary to raise water from a lower level to a higher one, it must be either carried up or pumped up through a pipe. In either case, work is done on the water, or in other words, energy is used. In the same way, heat always flows naturally from a region of higher temperature to a region of lower temperature. If the desire is to move heat from a region of lower temperature to a region of higher temperature, it is necessary to do work on it-to use energy. Therefore, the compressor in the cycle might be called a heat pump by means of which heat is pumped up-thermometer.
Figure 6-1. Mechanical refrigeration cycle.
Figure 6-1. Mechanical refrigeration cycle.
 
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In mechanical refrigeration, the compressor is, in effect a heat pump. In the compressor, work is done on the refrigerant by means of an electric motor which turns the compressor shaft, making the pistons move back and forth in the cylinders, The pistons compress the low-pressure vapor entering the cylinders to a high-pressure vapor.

Since, according to the law of conservation of energy, energy cannot be destroyed but can only be altered in form, it must appear somewhere. The input of energy has come through the electric motor by means of the crankshaft and pistons, and by their motion it is transferred to the vapor by increase in pressure. In that vapor, the input energy can appear only in the form of heat, which results in a rise in the temperature of the vapor.

6A3. The Freon 12 cycle. Let us follow through the cycle of operation, starting from the point where the heat to be removed enters the refrigerating system. This point is the location of the evaporator. Figure 6-1 is a highly simplified diagram of the main mechanical elements in the cycle.

6A4. Through the evaporator. The evaporator is simply a bank, or coil, of copper tubing. It is filled with Freon 12 at low pressure and temperature. Heat flowing from the air spaces or articles to be cooled into the coil causes the liquid Freon 12 to boil. Boiling can take place only as a result of the entrance into the liquid of its latent heat of vaporization, and this latent heat can come only from the surrounding substances. Hence the temperatures of the surrounding substances are lowered. The latter portion of the evaporator coil is therefore filled with Freon 12 vapor at low-pressure, carrying with it the unwanted heat.

6A5. Through the compressor. This vapor does not remain in the evaporator. The compressor is operating and the suction which it exerts (on the evaporator side of its circuit) pulls the heat-laden vapor out of the evaporator, through the piping, and into the compressor. The compressor, therefore, is the mechanism that keeps the Freon 12 in circulation through the system. In the compressor cylinders, the Freon 12 is compressed from a low-pressure vapor to a high-pressure vapor,

  and its temperature therefore rises, as explained in Section 6A2.

6A6. Through the condenser. The freon vapor, now at high pressure, passes next into the condenser, where the vapor passes around the tubes through which sea water is continuously pumped. Here the excess heat flows by conduction through the walls of the tubing from the higher temperature vapor to the relatively lower temperature sea water, and here the unwanted heat leaves the primary refrigerating system and is finally carried away. The excess heat thus flowing out of the vapor is both superheat and latent heat of vaporization, and therefore the vapor condenses back to the liquid state. The liquid Freon 12 is now at high pressure and high temperature.

6A7. Through the receiver. The liquid Freon 12 goes now into the receiver, or tank. The liquid in this receiver acts as a seal between the vapor in the condenser and the liquid as it flows into the next element, the expansion valve, so that the liquid Freon 12 in the expansion valve may be free of vapor. Remember that the whole system is a single circuit in which the fluid flows around and around.

6A8. Through the expansion valve. The liquid Freon 12 enters the expansion valve at high-pressure and high temperature. This valve regulates the flow of the refrigerant into the evaporator. The liquid outlet from this expansion valve is a small opening called the orifice. In passing through the orifice, the liquid is subjected to a throttling action, and there is dispersed into a finely divided form. The Freon 12 is now again a liquid at low pressure and low temperature, and is again entering the evaporator, its cycle completed, and ready to be repeated. Every part of the cycle is, of course, taking place simultaneously and continuously throughout the circuit as long as refrigeration is wanted. The entire operation is automatic.

6A9. Low-pressure side. That portion of the cycle from the orifice of the expansion valve through the evaporator up to and including the intake side of the compressor cylinders is called the low-pressure side. The dividing

 
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Figure 6-1a. Diagram of principle of refrigeration.
NavPers 17130, E-39, E-134
Figure 6-1a. Diagram of principle of refrigeration.
 
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line between the low- and high-pressure sides is the discharge valve of the compressor.

6A10. High-pressure side. The remainder of the cycle from the discharge valve of the compressor

  through the condenser, receiver, and expansion valve to its orifice is called the high-pressure side. The dividing line between the high- and low-pressure sides is the thermostatic expansion valve.
 
B. HEAT ACTION IN THE VARIOUS ELEMENTS OF THE CYCLE
 
6B1. In the evaporator. The evaporator is the point at which the heat from articles or air to be cooled enters the refrigerating system. This heat causes the Freon 12 to boil, and the rapid boiling carries tiny droplets of the liquid into the vapor. The Freon 12 at this stage is therefore a wet vapor. However, the design of the system is such that a little more heat is admitted to the evaporator than is required to produce saturated vapor. An additional superheat (about ten degrees) also enters the vapor in the evaporator. This super heat is kept fairly constant by the expansion valve. The superheat eliminates the wetness of the vapor, and prevents excessive frosting of lines and compressor and the possibility of carrying liquid over into the compressor. It also increases the efficiency of operation.

6B2. In the compressor. So far, the temperature of the boiling Freon 12 has not been raised (except for the slight superheat), because the heat entering it in the evaporator is latent heat of vaporization which serves only to turn the liquid into a vapor. But in the compressor, after the vapor has passed through the intake, or suction, valve, it is sealed off from its originating liquid. The heat now entering the vapor by the compression in the compressor, or heat pump, is more than sufficient to raise its temperature to the boiling point that corresponds to the new higher pressure. Thus the high-pressure vapor is further superheated.

6B3. Purposes of the compressor. The compressor serves several purposes

1. By suction, it removes the vapor from the evaporator as rapidly as it is formed so that there is always room for more vapor.

2. The steady suction tends to maintain a practically constant pressure in the evaporator; hence, the temperature of the refrigerant therein remains fairly constant.

3. It keeps fluid circulating in the system, thus maintaining continuous refrigeration.

  4. It compresses the low-pressure vapor to a high-pressure vapor, whereby the condensation point is raised to such a degree that the vapor can be condensed by the available cooling water.

5. It produces a difference of pressure on the two sides of the expansion valve, thereby causing a steady and positive flow of the refrigerant through that valve.

6B4. Necessity for the compressor. The compressor is an essential part of every mechanical refrigerating system. The question is often asked: Why is it necessary to compress refrigerants? Fundamentally, the reason is this: All the liquids used as refrigerants possess the peculiar property of boiling at low temperatures under atmospheric pressure. If one of these liquids were in an uncovered dish in the open air, it would boil briskly without any fire under it. Even on a cold day, the mere heat in the air is enough to make it boil. It was seen in Section 3D6, that the boiling point of a liquid varies with the pressure on it. By confining a liquid refrigerant in an airtight container (a refrigerating system is such an airtight container), we can increase or decrease the pressure on the liquid and thus place its boiling temperature at any degree we desire. By increasing the pressure on a vapor, we cause its condensation point to rise.

Now the refrigerant liquid in the evaporator has boiled at a low temperature. This low temperature vapor must give up its excess heat and condense back to the liquid state before it can be used again. Another such liquid cannot be used a second time for this purpose, for that would need still another, and so on endlessly. It is, necessary to use an easily available fluid, such as air or water. But the ordinary temperatures of water or air are considerably above the condensation temperature of the vapor. Fortunately, when a vapor is compressed, its temperature rises. Therefore,

 
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the vapor is compressed so that its temperature rises to such a degree above the ordinary temperature of the available water that the heat can transfer (in the condenser) from the now higher-temperature vapor to the water.

6B5. In the condenser. The excess heat enters the refrigerant while it is in the evaporator. Since the refrigerating system is a continuous circuit in which a given quantity of Freon 12 flows around and around, it is necessary to remove this excess heat before it can return to the evaporator. The condenser is the point at which this heat is removed.

In the condenser, the refrigerant passes around the tubes through which sea water is pumped. A fresh supply of sea water is continuously pumped through, and is not used again as is the primary refrigerant.

The unwanted heat transfers by conduction through the walls of the tubes from the refrigerant vapor to the sea water. More heat is discharged here than enters the refrigerant in the evaporator, for extra heat equaling the work done upon the vapor enters the vapor while it is being compressed.

It should be remembered that the temperature of the vapor, when it gets into the condenser, depends upon the temperature of the cooling medium, that is, the sea water. The compressor must produce a pressure on the vapor always high enough to make the vapor condense at the temperature in the condenser. Therefore; the condensing temperature that exists in the condenser determines the minimum discharge pressure of the compressor.

Naturally, the sea water inside the condenser does not remain at the same temperature. The heat leaving the vapor enters the water, and the water temperature rises. How ever, the continuous flow of water through the tube prevents this rise from getting too high. Actually the rise within the condenser is only a few degrees.

The heat transfer inside the condenser, therefore, is as follows: First the temperature of the vapor drops as the superheat leaves it. When it reaches the condensation point corresponding to the pressure, the vapor becomes a saturated vapor, and as such, if further heat is removed, it must condense. The temperature

  remains constant while the latent heat is departing during condensation. The condensed liquid, in the bottom of the condenser shell or tank, is not in contact with the entering section of the tubing through which the water flows. The heat in the refrigerant liquid is now sensible heat and therefore it drops again in temperature by a small additional amount. The whole operation within the condenser is a constant-pressure process.

a. Necessity for subcooling in the condenser. Subcooling, or lowering the temperature of the liquid refrigerant below its saturation, or boiling, point, is essential before it reaches the expansion valve. This subcooling is necessary to insure that vapor mist or vapor bubbles are not contained in the condensed liquid. If the condensed liquid were allowed to remain at its saturation temperature probably only vapor bubbles would be present, either as a result of being carried uncondensed from the compressor vapor, or as a result of some slight evaporation in the line from the receiver to the expansion valve. Subcooling in the condenser prevents this possibility.

1. The orifice in the expansion valve is designed to pass the correct amount of liquid refrigerant to furnish the desired cooling. If vapor mist is mixed with the liquid, a smaller amount of liquid passes through the expansion valve, and the system cannot produce its full rate of refrigeration.

2. Refrigeration is produced only by the alternate evaporation and condensation of the refrigerant. If any vapor passes around the cycle uncondensed, it produces no refrigeration, and the energy used in pumping it through the system is wasted.

Thus it is evident that the subcooling in the condenser plays a most important part in the efficient operation of the system.

6B6. In the receiver. No heat action takes place within the receiver, that is, the receiver plays no part in the heat cycle. It serves as a momentary storage for the liquid refrigerant that leaves the condenser, and as a seal between the vapor in the condenser and the liquid as it flows into the expansion valves. In some types of condensers, the bottom part of the shell or tank is used as a receiver.

 
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6B7. In the thermostatic expansion valve. The purposes of this valve are 1) to control the quantity of liquid refrigerant passing into the evaporators; 2) to maintain a constant pressure on the refrigerant so that the super heat is held practically constant regardless of the suction pressure; 3) to disperse the liquid; and 4) to prevent the liquid from surging toward the compressor. The total heat present remains constant.

a. Theory of operation. Consider a refrigerant evaporator in an air-conditioning unit operating with Freon 12 at 37 psi suction pressure. The Freon 12 temperature at saturation at 37 psi is 40 degrees. As long as any liquid exists at this suction pressure, the temperature remains at 40 degrees.

Freon 12 moving along within a coil absorbs heat from the air outside the coil until (B, Figure 6-2) it has absorbed its latent heat of evaporation. At this point all the liquid has evaporated and the vapor is saturated. Any additional heat absorbed from the surrounding air raises the temperature of the vapor but the pressure remains at 37 psi. When the suction gas or vapor reaches the point of the thermal bulb attachment (C), it is superheated according to the thermal valve setting; for example, 10 degrees.

Neglecting heat transfer loss from the suction line to the thermal bulb, the temperature of the liquid Freon 12 within the bulb is 50 degrees, the temperature of the suction gas at this point. The pressure within the bulb, and consequently within the power assembly, is 46.7 psi (P1). This force tends to push the valve diaphragm down, opening the valve. Opposing this force is the pressure, 37 psi (P2), with the evaporator at 40 degrees evaporator temperature, and the force (Ps) exerted by the spring on the diaphragm. To keep the valve in equilibrium at 10 degrees superheat, this spring is externally adjusted to exert a force of 9.7 psi on the diaphragm.

  b. Thermostatic expansion valve action. If the superheat in the suction gas increases, as in the case of an increase in load, the thermal bulb temperature and its corresponding pressure increase, exerting a greater pressure on the diaphragm. This causes the valve to open to allow a sufficient increase in flow of refrigerant to restore the superheat to 10 degrees. If the superheat decreases because of a falling off in the load, the pressure in the thermal bulb, and consequently, in the power element, decreases and tends to close the valve. The flow of refrigerant is throttled enough to increase the superheat to 10 degrees. Thus, it is evident that the function of the thermostatic expansion valve is twofold: 1) automatic expansion control, and 2) prevention of the liquid refrigerant from surging back to the compressor.

Close control of superheat results in the greatest coil efficiency. However, superheat should never be maintained below 5 degrees because of the danger of the liquid refrigerant surging back to the compressor. Nor should a coil be operated with superheat in excess of 15 degrees because of the inefficiency of operation beyond that point.

Figure 6-2. Superheat action In evaporator.
Figure 6-2. Superheat action In evaporator.

 
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