Vapor-compression heat pumps

There are two main types of heat pumps vapor-compression pumps and absorption pumps. Vapor-compression heat pumps are the more common of the two. 

More insight is yet available from the data in Table II. In the refrigerated process, the two condensers and the throttle valve involve more than 50% of the lost work remaining. One way to eliminate the inefficiencies of the condensers is to recycle the latent heat of the overhead vapor in a heat pump (vapor recompression) system, as shown in Figure 3. The distillation tower pressure, and hence its overhead temperature are kept the same, but the overhead, instead of being condensed, is compressed to a pressure at which it will condense at 77°F (about 180 psig). 

Conventional, (Fig. 17.8) Heat Pump with External Refrigerant, (Hg. 17.11) Heat Pump with Compression of Overhead Vapor, (Fig. 17.12) Heat Pump with Reboiler Liquid Rashing, (Fig. 17.13)... 

Figure 17.10. Use of heat pumps in distillation, (a) Heat pump with external refrigerant, (h) Heat pump with compression of overhead vapor, (c) Heat pump with reboiler liquid flashing. [H. R. Null, Chem. Eng, Progr., 72 (7), 58-64 (1976).]...

Figure 17.12. Low-temperature distillation using heat pump with compression of overhead vapor for separation of propylene-propane system...

A schematic of a simple vapor compression heat pump is shown in Fig. 6.37. A heat pump is a device that absorbs heat at a low... 

FIG. 13-2 Complex distillation operations with single columns, a) Use of intermediate heat exchangers, (h) Coupling of intermediate heat exchangers with heat pump, (c) Heat pump with external refrigerant, (d) Heat pump with vapor compression, (e) Heat pump with hottoms flashing. 

Figure 16.36 Schematic of a simple vapor compression heat pump.

The actual vapor heat pump cycle deviates from the ideal cycle primarily because of inefficiency of the compressor, pressure drops associated with fluid flow and heat transfer to or from the surroundings. The vapor entering the compressor must be superheated slightly rather than a saturated vapor. The refrigerant entering the throttling valve is usually compressed liquid rather than a saturated liquid. 

In the air-conditioning mode, a domestic air-conditioning and heat-pump system as shown in Fig. 6.22 uses R-134a as the refrigerant. The refrigerant-saturated vapor is compressed from 140 to 700 kPa. Summer ambient air at 30°C is to be cooled down to 17°C. Find the compressor power required, heat removed from the ambient air in the heat exchanger, COP of the system, and mass rate of air flow per unit of mass rate of refrigerant flow. 

The main factor that has been responsible for the slow adoption of absorption heat pumps for heating and air conditioning duties has been their high capital cost compared with that of vapor compression equivalents. This is due largely to the cycle complexity, as shown in Figure 14, which displays the four principal cycle elements, all of which involve vapor-liquid systems ... 

In the aforementioned process, the heat for the reboiler is usually available as waste heat from the steam cracker, for example, and is essentially cost-free. If this heat is not available, a heat pump can be used. The heat pump can upgrade the heat, at an exergetic cost, to the desired temperature level. If the separation is viewed in isolation, this means that the heat rejected by the condenser at relatively low temperature, can be upgraded to be the higher temperature heat input for the reboiler. A schematic of the heat pump process is given in Figure 10.2. The overhead vapors are heated slightly in the reflux subcooler, which enables these vapors to be compressed and cooled in the condenser-reboiler. 

F A saturated vapor stream of n-butane is available at 2.2 atm for a heat pump application. The vapor is compressed to 4 atm and 124°F. The heat released when it is condensed to a... 

For example, the reboiler and condenser from the same column may be heat-integrated by using a heat pump, as discussed by Null [Chem. Eng. Prog., 72(7), 58 (1976)]. A heat pump may use an external fluid which is vaporized in the condenser, then compressed and condensed in the column reboiler (Fig. 13-75 ). Another heat pump (Fig. 13-75b) uses column overhead vapor, which is compressed and condensed in the reboiler and then returned to the top of the column as reflux. A third possibility is to use the column bottoms, which is let down in pressure and vaporized in the condenser, then compressed and fed back to the bottom of the column as boil-up (Fig. 13-75c). 

Cl Cost of high-grade energy supplied to the compressor of a vapor compression heat pump /GJ In (a)... 

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