Vapor compression cycle heat pump

Fig. 7.21 Conventional vapor compression cycle heat pump (left) compared with absorption cycle heat pump (right).

The vapor-compression cycle was first used by French engineer Nicolas Leonard Sadi Carnot in 1824. Then in 1832, American inventor Jacob Perkins was the first to demonstrate a compression cooling technology that used ether as a refrigerant. But it was in 1852 that Scottish engineer William Thomson, also known as Lord Kelvin, conceptualized the first heat pump system, dubbed the heat multiplier. ... 

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. 

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 ... 

Heat pumps (closed cycle) Vapor compression Better instrumentation and control Optimization of dryer design and operation Improved dewatering of feedstock... 

The principle of the heat pump, which is the same as that involved in the refrigeration operation, has been known for over 100 years. In the last four decades, heat pump applications have been limited only by economics. Among many types of heat pumps (e.g., vapor compression, absorption, ejector, Brayton cycle, and thermoelectric), only the first found wide application because of its high effideucy and relatively simple construction. 

Conventional heat pump operation is indicated in Figure 16-1. A refrigerant, upon boiling, absorbs the heat that would otherwise be rejected in a condenser. The refrigerant vapor is compressed to a pressure adequate to permit the vapor to be condensed in the calandria, thereby providing the heat needed for evaporation,. The condensate from the calandria is flashed into the condenser thereby completing the cycle. 

These heat pump systems are the vapor recompression heat pump (VRHP) at which the top vapor of the column is compressed and then boils up the bottom liquid in a heat exchanger, the bottom flashing heat pump (BFHP) where the bottom liquid has been flashed through a valve and then condenses the top vapor by exchanging the heat, and the absorption heat pump (AHP) with a cycle of water/ ammonia working pair. 

There are several types of heat pumps that are currently feasible for industrial applications, including mechanical vapor-compression systems, closed-cycle mechanical heat pumps, absorption heat prnnps, heat transformers, and reverse Brayton-cycle heat prnnps. 

Radermacher, Reinhard, and Wmho Hwang. Vapor Compression Heat Pumps with Refr erant Mixtures. Boca Raton, Fla. GRG Press, 2005. The author provides a more advanced examination of refrigerant mixtures and working fluids, including thermodynamic aspects, refiigerant cycles, and heat transfer. 

In order to achieve the isothermal heat addition and isothermal heat rejection processes, the Carnot simple vapor cycle must operate inside the vapor dome. The T-S diagram of a Carnot cycle operating inside the vapor dome is shown in Fig. 2.2. Saturated water at state 2 is evaporated isothermally to state 3, where it is saturated vapor. The steam enters a turbine at state 3 and expands isentropically, producing work, until state 4 is reached. The vapor-liquid mixture would then be partially condensed isothermally until state 1 is reached. At state 1, a pump would isentropically compress the vapor-liquid mixture to state 2. 

By means of evaporation, dissolved pollution is concentrated with the aim of obtaining distilled purified water from wastewater. In mechanical vapor recompression (MVR), the influent is inserted in the system, where it is distributed across heat elements and as a consequence is partly evaporated. This vapor is compressed by a compressor and is then transported to the inner surface of the heat element where it condenses and is collected. The concentrated wastewater is deposited onto the bottom of the device and is subsequently transported by the concentrate pump, after which the cycle starts all over. The technique is effective (ca. 99%), which is dependent on the influent and the type of pollution. 

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