Free cooling

Air-water systems (e. g. fan coil) require a supply of chilled water at a temperature of about 10°C throughout the year. The methods used for this purpose are:

(i) Thermo-syphonic cooling. This was dealt with in section 9.10. The refrigeration plant is operated in winter, with its compressor shut down, to produce chilled water at a suitable temperature by a natural cooling process. An air-cooled condenser or a cooling tower is used.

(ii) A cooling tower can give water at a suitable temperature in winter, when the outside air wet-bulb is sufficiently low. The chilled water from the tower by-passes the condenser and is fed directly into the secondary circuit to the fan coil units. The chilled water produced is dirty because the circuit is open and subject to pollution from the atmosphere, and to corrosion. The larger particles of dirt are removed by a good quality water filter and the fan coil or other units fed with the water must be able to function with the level of contamination remaining. The pH of the water is monitored as a check on corrosion. Motorised, automatic valves direct the water to the condenser in summer (when the refrigeration plant must run) and to the units in winter (when the refrigeration plant is off). When using this method it is important to check the hydraulic balance between the chilled water and cooling water circuits: these are hydraulically separate in summer but interconnected in winter.

(iii) If the water fed to the units must be clean, as is often the case, then a plate heat exchanger is inserted in the cooling water circuit. This transfers heat indirectly between the cooling water and secondary chilled water circuits for the fan coil units. The secondary chilled water flow temperature will be a degree or two warmer in winter, because of the temperature difference required at the heat exchanger. The advantages are that the secondary chilled water circuit is clean and the cooling and chilled water circuits are hydraulically independent.

(iv) A dry cooler may be used. In simple terms this is a free-standing cooler coil over which outside air is drawn or blown, by a fan. A mixture of water and glycol is circulated through the coil, naturally chilled, and fed to the units. Owing to the viscosity and other physical properties of the aqueous glycol mixture (used as an anti-freeze measure) the thermal resistance of the surface film within the tubes is increased. The pressure drop through the system is also affected and appropriate allowances must be made for these effects when selecting the dry cooler, the fan coil or other units, and the pumps. Since only sensible heat transfer occurs, the surface areas needed for heat transfer in dry coolers tend to be rather large. The method is really only suitable for smaller installations.

(v) Cooling at the primary air cooler coil. If the primary chilled water pump is kept running but the refrigeration plant is switched off, water will be circulated through the primary air cooler coil. When the outside air dry-bulb is less than about 5°C the flow of cold outside air over the cooler coil can chill the water pumped through its tubes to about 11°C or 12°C, which the secondary pump then circulates to the units. The freezing risk, when using the primary cooler coil in this way, must not be ignored.

(vi) In the past, sprayed cooler coils were used in a manner similar to that outlined in (v), above. These were more effective in producing chilled water for secondary circuits because of the evaporative cooling effect of the spray water. The method has fallen into disuse because of the associated hygienic risks.

Posted in Air Conditioning Engineering

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