Refrigeration Cycles without Refrigerants

A conceptual breakthrough in one process can be adapted to other processes. If the freezer desalinator can be modified to operate without a refrigeration loop and a coolant, perhaps an air conditioner can be modified as well 

The air warms in the adsorber because water vapor releases heat when it adsorbs. Of course, the desiccant would be recycled, for example, by heating to drive off the water. The coolant would be recycled through a refrigeration loop, such as the loop we developed for the desalination process. 

Most air conditioner refrigerants were chlorofluorocarbons (CFCs) until it was discovered that fugitive CFCs catalyze the destruction of ozone in the upper atmosphere. One solution is to replace CFCs with less-harmful substances. Another approach is to eliminate the need for a coolant by using the process fluid (air) to pump the heat. 

Engineering the desiccant loop is challenging because it is not easy to circulate a solid. Two designs for the desiccant loop have been reported. The Albers Corporation redefined the problem of pumping a solid by developing a liquid desiccant. ICC Technologies and LaRoche Chemicals each have designs in which the desiccant is a solid wheel on an axis parallel to the adsorber and the dryer. As the wheel turns, the desiccant moves between the two units. 

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The technical reasons for the unavoidable rejection of heat at the lower-temperature reservoir will become clearer when we consider actual cycles in Chanter 6. The fundamental reason, however, is evident in Figure According to eg. (4. o). the amount of heat Ql is equal in absolute value to the area DCC D. Clearly, it is impossible to complete the cycle without rejecting this amount of heat at the lower reservoir. The area is zero only if Tl is equal to absolute zero, and in this case the Carnot efficiency becomes i. For any other value of Tl, the amount of Ql is nonzero and the efficiency of the cycle less than i. In practice, Tl is fixed by the temperature of the ambient surroundings. It is possible of course to produce temperatures lower than ambient using refrigeration, but we will see in Chapter 6 that this requires work and cancels the benefit from the higher efficiency of the cycle. 

Problem 6.38 The refrigeration cycle in Figure 6-21 shows a vapor compression cycle with recirculation. The vapor-liquid separator after the throttling valve recycles the vapor to the compressor and passes the liquid to the evaporator. This process uses the refrigerant R-i34a with the evaporator temperature set at - 20 C, the condenser at 30 "C, and with compressor efficiency of 75%. Determine the energy and entropy balances and compare the efficiency of this process to the simple vapor-compression cycle without recirculation. 

With this aim, Carnot proceeded to introduce the novel conception of a reversible cycle of operations, and arrived at the exceedingly important result that the motive power capacity for doing wrork] of heat is independent of the agents employed to develop it its quantity is determined solely by the temperatures of the bodies between which, in the final result [i.e., after the completion of the cycle], the transfer of heat occurs. Let there be given a source, and a refrigerator, at temperatures Ti, T2 respectively, where Tx > T2. In order that finite quantities of heat may be added to or taken from these without change of their temperatures, we may suppose them to consist of... 

A heat pump is a device that operates in an opposite manner to a heat engine takes the amount of heat Q2 from a source at T2 and delivers an amount Qi to a source at T by spending W mechanical energy. Without losses the heat balance indicates that Q =Qj + fV Classical examples are the domestic refrigerator, or the air-conditioning system. A heat pump can operate as a reversible cycle, so that Eq. (11.3) still holds. 

Fig. 7. Miniature Raman cell for frozen protein solutions that, once loaded with sample, can conveniently be shipped between laboratories in dry ice or liquid Nj and then attached without further manipulation to a helium closed-cycle refrigerator (CCR) station for RR measurements. The main advantages of this design are as follows (1) very small quantities of sample are required (1-2 drops) (2) the cell atmosphere can be controlled (3) cryogenic temperatures are obtained, down to 10 K and (4) Raman scattering originates directly from the surface of a frozen solution without interferences from glass or quartz scattering. ...

Closed Cycle - A system in which a working fluid is used over and over without introduction of new fluid, as in a hydronic heating system or mechanical refrigeration system. 

The minimum required work corresponds to reversible operation (Sge = o). If the cycle is irreversible, it will require more work. Positive work means that we (the surroundings) must supply this amount to the device. This is precisely what a refrigerator does It uses work to pump heat from lower to higher temperature (more about refrigeration in Chapter 6). Without supplying work from the surroundings, the process is impossible. This proves the Clausius statement. 

This cycle is repeated over and over again until the temperature reaches the desired level (about 1-2°C in the food chamber of a domestic refrigerator and minus 15-18 C in the deep-freeze compartment). The compressor is then switched off, and on again later, by a thermostat to maintain a steady temperature, hi order to transfer heat from the cold interior to the warm surroundings without contravening the second law of thermodynamics, energy has to be supplied to the cycle by the electric current that drives the compressor. 

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