How It Works: Science and Technology - Vol. 8

By Wendy Horobin | Go to book overview

Heat Pump

It is a basic law of nature that heat will flow from a hot body to a colder body, but not the reverse. With a heat pump, however, this reverse operation becomes possible: that is, heat is taken from the colder body and pumped to the hotter body. In the process, the hot body becomes hotter and the cold body becomes colder.

A heat pump does not violate any laws of nature however; to achieve this end, power must be supplied to the pump to make it work. It therefore operates like a heat engine in reverse.

A heat engine operates between a hot body and a cold body (these are usually called reservoirs), and part of the heat, as it flows from the hot reservoir to the cold reservoir, is converted into mechanical work. To conceive the reverse of this process, imagine work being put into the engine and heat flowing in the opposite direction. This is the basic principle of the heat pump.

A heat pump is therefore identical in operation to a refrigerator—differing only in purpose. A refrigerator is used specifically to cool something down still further by removing heat from it, whereas the heat pump places the emphasis on where the heat is going to—namely, increasing the temperature of the already warm body.


Working principles

The term heat pump is somewhat misleading since heat is not a fluid like air or water and cannot in fact be pumped—except in a metaphorical sense. Some medium is required to make it possible for this transfer of heat to take place.

A heat pump functions rather like a steam power plant (which is a heat engine) working backward, except that steam is unlikely to be the working fluid. A vapor or refrigerant, such as ammonia, carbon dioxide, or a halocarbon, is more likely. The fluid flows around a closed circuit driven by a compressor or pump. The fluid enters the pump as vapor and experiences a rise in pressure and temperature. The vapor then enters the condenser (which is a heat exchanger), and heat is transferred to the warm reservoir, which is cooler than the vapor entering the condenser. Here, the vapor condenses and leaves as a liquid, still at a high pressure.

The liquid then flows through an expansion valve—a restricted passage through which the liquid spurts into a low-pressure area. This reduction in pressure causes the liquid to vaporize partially and is accompanied by a reduction in temperature. The liquid-vapor mixture now flows through the evaporator, which is situated in

A type of heat pump
used in air-conditioning
plants. In warm weather,
it provides a cooling draft.
In the winter, it can be
converted to extract heat
from the outside air to add
to the heating system.

the cold reservoir (this is the second heat exchanger in the system). Because the liquidvapor mixture is now colder than the cold reservoir, it takes heat from the reservoir and moves on to the compressor at a higher temperature than when it went in. This is the complete cycle.


Coefficient of performance

A typical application of a heat pump might be to take heat from a cold reservoir, such as a river, and transfer it to a building that requires heating (this is the warm reservoir). In the process, work or power must be supplied to the pump to drive it. The heat pump might, for example, receive 1,000 kJ from the river, absorb 400 kJ in power from an external source to drive it, and deliver 1,400 kJ in heating to the building.

The performance of the heat pump is measured by the ratio of heat delivered to the warm reservoir to the work absorbed by the pump to drive it. For the above example this ratio is 1400/400, which works out to 3.5; this figure is called the coefficient of performance.

The ideal coefficient of performance for a heat pump is that of a reverse Carnot cycle working over the same temperature range. If the hot reservoir is at temperature T1 (degrees absolute) and the cold reservoir at temperature T2, then the maximum coefficient of performance is given by T1/(T1 - T2). Thus, for a hot reservoir temperature of 77°F (25°C, 298 K) and a cold reservoir temperature of 36°F (2°C, 275 K), the ideal coefficient of performance is given by 298/(298 - 275)

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How It Works: Science and Technology - Vol. 8
Table of contents

Table of contents

  • Title Page *
  • Contents *
  • Gold 1013
  • Governor 1017
  • Grass-Cutting Equipment 1018
  • Gravity 1020
  • Gun 1023
  • Gyrocompass 1028
  • Gyroscope 1030
  • Hair Treatment 1032
  • Halogen 1034
  • Hang Glider 1037
  • Head-Up Display 1039
  • Hearing 1041
  • Heart 1045
  • Heart Pacemaker 1048
  • Heart Surgery 1049
  • Heat Engine 1053
  • Heat Exchanger 1054
  • Heating and Ventilation Systems 1056
  • Heat Pump 1063
  • Helicopter 1065
  • Hi-Fi Systems 1071
  • High-Speed Photography 1077
  • Holography 1080
  • Hormone 1084
  • Horticulture 1088
  • Hosiery and Knitwear Manufacture 1090
  • Hurricane and Tornado 1094
  • Hydraulics 1100
  • Hydrocarbon 1105
  • Hydrodynamics 1109
  • Hydroelectric Power 1112
  • Hydrofoil 1116
  • Hydrogen 1118
  • Hydroponics 1120
  • Hygrometer 1123
  • Ignition System, Automobile 1124
  • Image Intensifier 1128
  • Immunology 1132
  • Induction 1138
  • Inertia 1142
  • Information Technology 1147
  • Ink 1151
  • Index i
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