Elementary Refrigerator

In Fig. 1-19, refrigerant cylinder A is shown with the valve closed. The pressure, the temperature (inside and out), the num­ber of molecules leaving the gaseous state, the number diving back into the liquid, and the liquid molecules flying out of the liquid into the gaseous state are equal. All con­ditions are balanced.

On cylinder B, the valve has been opened slightly and some of the gas is escaping. The results are twofold. The pressure over the liquid refrigerant in the cylinder is reduced. This will cause more of the liquid to change to a gas. In changing from a liquid to a gas, heat is absorbed and the liquid refrigerant will be cooled. The tem­perature of the refrigerant and cylinder is now 50 F. Some heat from the surrounding area, which is at 70 F., will now flow into the cylinder and into the refrigerant.

Fig. 1-19. The cooling effect of different pressures operating on the surface of liquid refrigerant R-12. A-Shows a cylinder of refrigerant with the valve closed. The refrigerant is in a state of equilibrium. The refrigerant and the cylinder are both at room temperature. The pressure in the cylinder is 70.5 psi. B-Shows a cylinder of refrigerant with the valve slightly opened and refrigerant gas beginning to leave the cylinder. In order to maintain an equilibrium between the gas leaving the liquid and the internal gas pressure the liquid absorbs heat from the cylinder and the surrounding air. This has caused the temperature to drop to 50 F. The pressure in the cylinder.is 47 psi. C-Shows the cylinder of refrigerant with the valve open wider. The refrigerant gas is flowing rapidly from the cylinder. A large amount of heat will be absorbed since the pressure on the surface
of the liquid is not great enough to retard the vaporization of the refrigerant. The rapid absorption of heat has reduced the refrigerant and cylinder temperature to 5 F. This results in rapid absorption of heat from the surrounding space. The pressure in the cylinder is 12 psi

On cylinder C, the valve has been opened more than at B with the result that refrigerant gas flows out more rapidly. This results in still a lower pressure on the liquid refrigerant and more rapid evaporation of the refrigerant. This in­crease in the rate of evaporation lowers the temperature of the refrigerant and the cylinder still more with the result that the 72 F. air surrounding the cylinder will more rapidly give up its heat to the colder cylinder.

On cylinder A, we have a state of equilibrium with al I temperatures and pres­sures in balance.

On cylinder B, we have a slight un­balance due to the gas escaping through the valve. If this condition were to continue for a considerable time, a condition of balance would again prevail but in this case, its balance would not be a static one as in A; rather. a balance between the rate of heat flow into the cylinder, the evapo­ration of refrigerant, and the flow of re­frigerant gas out of the cylinder valve. In this condition of balance, the refrigerant is cooling the cylinder and its surroundings.

On cylinder C, we have a greater un­balance than at cylinder B with the result that the cylinder pressure and temperature will be lowered still further.

As long as the valve is open and gas molecules can escape, the temperature will be lower because more liquid molecules are becoming gas molecules, then gas molecules are returning to the liquid. This gas bombardment is called vapor pressure. If this vapor pressure can be reduced, the temperature of the liquid can be re­duced since evaporation will be increased.

If the gas molecules can be removed fast enough by any means such as suction from a compressor, a chemical to absorb the molecules, or other means, a low enough vapor pressure may be produced to create refrigerant boiling temperatures which are at the refrigerating level.

Fig. 1-20. An elementary refrigerator using R-12

 but not recondensing the vaporized refrigerant.

 A-Re­frigerated space. B-Cylinder of liquid refrigerant.

 C-Evaporator. D-Control valve. E-Pressure gauge.

The operation of the mechanical re­frigerator is based on the heat absorption property of a fluid passing from the liquid to the gaseous state. If one were to put a cylinder of refrigerant into an icebox and vent the gas to the outside, we would have a heat absorber in the box, as shown in Fig. 1-20. The liquid can boil only at its evaporation temperature, say 20 F., and this liquid will be at this temperature until it has completely evaporated. If one tried to raise its temperature by adding heat, the only result would be a more rapid evaporation of the liquid into a gas, provided the pressure remained constant. Being at this low temperature there is, of course, a transfer of heat to it from the surround­ing objects. This heat helps the evaporating and the heat is carried away in the vapor passing off. Thus, the fluid changing its state to gas gets the energy (or heat) for doing this from the objects surrounding it, and that heat is removed with the vapor to the outside of the box.

This type of refrigerator works nicely, but it is an expensive method because the refrigerant fluid is lost. There are some mobile refrigerating units (trucks) that use this method. The refrigerant used is usually liquid nitrogen which is relatively inexpensive. It is called chemical re­frigeration.

In the mechanical refrigerator, this es­caping vapor is captured, compressed, and cooled to a liquid state again so that it can be used over and over, as shown in Fig. 1-21.

To follow through the refrigerant cycle in this refrigerator mechanism, begin with the refrigerant in tank B. The re­frigerant used in this illustration is R-12.

The refrigerant at B is under a pressure corresponding to the room temperature of 72 F. For R-12 this pressure will be approximately 71 psi. At the refrigerant control, C, this pressure is reduced to provide low-pressure, low-temperature evaporation in the evaporating (cooling) coil. Since the refrigerant temperature is to be held at 35 F. the pressure in the evaporating coil must be held at or below 30 psi. It is the purpose of the refrigerant control to allow refrigerant to flow from the storage tank (high side) into the evapo­rator coil (low side) while at the same time maintaining the pressure difference be­tween the high-pressure side (high side) and the low-pressure side (low side). In evaporator, D, the liquid refrigerant is now under a much-reduced pressure and it will evaporate or boil very rapidly thus cooling the evaporator coil.

Fig. 1-21. An elementary mechanical refrigerator. A-Refrigerator. B-Liquid refrigerant receiver. C-Refrigerant control. D-Evaporator. E-Motor driven compressor. F-Condenser. In this refrigerator, the refrigerant is not allowed to escape but is recycled as necessary to obtain the desired temperatures.

The compressor, E, draws (sucks) the evaporated refrigerant gas from the evaporator coil and com­presses it back to the high side pressure. From the compressor, the high temperature (see heat of compression, Par. 1-33) high-pressure gaseous refrigerant flows into the condenser coils F. The temperature of the gas as it enters these coils will be several degrees warmer than the room temperature. This results in a very rapid transfer of heat from the condenser coil to the surrounding air. The gas as it flows through the condenser is cooled and loses its heat of evaporation and re­turns to the liquid state again. As a liquid it flows from the condenser coils back into the liquid refrigerant storage at B. This refrigeration cycle is repeated over and over until the desired temperature is reached and a thermostat breaks the circuit to the driving motor and the compressor stops.

The temperature at which a refrigerator cooling unit is kept depends on the pres­sure at which the refrigerant is evaporated, while the amount of heat removed depends entirely on the amount of refrigerant changed into a gas.