地源热泵系统热力特性分析外文翻译资料

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Function

Mechanical design based on working media

• • 1. Heating coils

Used to heat the supply air to the required temperature (e.g. the discharge air tem- perature of a room heating system).

• Finned-pipe heat exchanger, operated with low temperature hot water, high tem- perature hot water or refrigerant vapor (Fig. 6-19)
• Electric heating coils (Fig. 6-21)

Heating coils are used as pre-heaters or re-heaters in air heating and air condition- ing plants.

Fig. 6-19 Finned-pipe heating coil and example of installation in a plant (with a mixing circuit)

Heat exchangers normally show a non-linear response in terms of the mass flow passing through them and the associated output. Depending on how they are con- structed and on the supply temperatures, this 'heat transfer characteristic' varies from a steep curve to a shallow curve, as expressed by the 'a-value'. The charac- teristic curve of the heat exchanger is shown in relation to the maximum volume

flow rate Vamp; 100 and the maximum output Qamp; 100 .

Fig. 6-20 Heat-exchanger characteristic curves and the associated a-values

Electric heating coils have a number of built-in spiral heating elements which be- come hot as the current flows through them. They emit the heat acquired in this way into the air. They are installed in areas where there is no hot water heating, or where the connection point is too far away, or where, for other reasons, an LTHW heating coil cannot be used. Owing to the risk of fire, electric heating coils require special safety arrangements and devices (e.g. safety thermostat, fan run-on etc.)

Fig. 6-21 Electric heating coil (steel pipes with integrated heating coils; Source: Loysch)

6.2.6 Chilled water cooling coil

Chilled water cooling coils are finned-pipe heat exchangers operated with flow/return water temperatures of e.g. 6/12 °C or 8/14 °C. Cooling coils normally require a larger heat-transfer surface area than heating coils, because on average, the difference in temperature between the surface of the cooling coil and the air is smaller. The requirement for a greater surface area is fulfilled in design terms by several rows of pipes arranged one behind the other. If the air is to be de- humidified as well as cooled, then the cooling coil must be connected hydraulically as shown below, i.e. in a diverting circuit. If the air only has to be cooled, then a mixing circuit is acceptable.

2

1

Fig. 6-22 Example of installation of a chilled water cooling coil in the plant (with diverting circuit)

1. Chilled water inlet
2. Air inlet

6.2.7 Direct expansion cooling coil

A finned-pipe heat exchanger which acts as the evaporator in a refrigerant cycle is installed directly in the air flow as a cooling coil. As a rule, this solution is reserved for compact cooling units which also have a built-in compressor and condenser.

Fig. 6-23 Direct expansion cooling coils Various types (left)

Detail showing connection of the refrigerant distributor (right)

6.2.8Humidifiers

Principles

Air washers

Water atomizers

• Humidification through the evaporation of water
• Humidification through the introduction of steam
  • • 1. Evaporative humidifiers

Evaporative humidifiers include air washers, water atomizers and surface water evaporators.

Water is pumped from a reservoir into the spray nozzles distributed in the air flow. The majority of the fine droplets evaporate and become steam, in the process of which the latent heat of vaporization is extracted from the air flow. Since the energy required for evaporation is taken solely from the air, the air cools down (adiabatic cooling). The non-evaporated water droplets are separated in a drip screen at the air washer outlet, and then routed back to the collecting tray.

Fig. 6-24 Principle of the air washer, and example of construction (Source: Baehr)

Fig. 7-15

1. Spray nozzles (several adjacent rows, often with the spray directed against the air flow)
2. Drip screen
3. Collecting tray
4. Circulating pump

The water is converted by use of atomizing nozzles into a fine spray mist. The wa- ter droplets or 'aerosols' are so small that they are initially suspended in the air, and then evaporate completely. The latent heat of vaporization is removed from the air, which becomes slightly cooler (adiabatic cooling).

Fig. 6-25 Surface water evaporator (built into ventilation unit)

Surface water evaporator

Cold water steam humidifier

Porous ceramic plates with a large surface area are located downstream of the wa- ter atomizer described above. Any aerosols which have not yet evaporated are trapped by these plates and then evaporate fully (see Fig. 6-25).

In plants required to satisfy stringent hygiene requirements, cold water steam gen- erators can be used for humidification. Compared with the methods of humidifica- tion described above, the characteristic features of a cold water steam generator are a much lower consumption of water and energy.

The incoming air is f

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