The UK climate has traditionally not required the use of cooling systems for comfort purposes since the average external temperatures are lower than, say, mainland Europe or elsewhere in more southern climes. However, modern buildings tend to have larger areas of glazing (particularly south-facing), greater emphasis on improved U-Values for external envelope components and lower air infiltration rates. The result of these factors is that without the use of cooling systems, the internal heat gains tend to cause overheating of buildings in warmer weather. For isolated areas, DX-type air conditioning is used typically with an internal unit mounted in the ceiling and an external condenser on the roof or in an external compound. In larger or industrial applications it is more normal to use a water-based cooling loop with a relatively large water or air-cooled chiller located again either on the roof or at ground level in a plant compound. Traditionally, there have been two main temperature ranges used for cooling or chilled water systems as follows:
- High Grade Chilled Water (HGCW, 6-12DegC, dT 6DegC)
- Low Grade Chilled Water (LGCW, 14-17DegC, dT 3DegC)
There are of course many other temperature ranges and Differential Temperatures (dTs) in use today but the above temperatures are the most common.
The HGCW systems are usually used when large heat transfer rates are required and condensation is not of particular importance. Where condensation needs to be avoided either for psychrometric reasons or the risk which the condensate would present to, for example, expensive IT equipment, then Low Grade Chilled Water systems are used where the flow and return temperatures are above the dewpoint of the air in which they come in contact.
The lower limit of 6DegC for the flow temperature used in HGCW systems is usually dictated by the practical operating limits of the chiller plant and the economics of running with various concentrations of glycol mixtures. This is not an absolute limit and using specialist models of chillers with high concentrations of glycol temperatures of -10DegC can be achieved for example. This temperature is required for bespoke cooling circuits normally associated with process rather than comfort. The upper limit of 12DegC is somewhat dictated by the narrowing Log Mean Temperature Difference (LMTD) between the chilled water temperature and the temperature of the medium to be cooled. When the LMTD reduces, the area of the exchange surface must rise exponentially and therefore practicality becomes the limiting factor.
As with heating systems, cooling systems have also traditionally operated as constant flow systems with 3-port control valves varying the flow rate through the emitter with the unused portion of the flow sent back to the return. With a constant flow rate, the dT reduces with decreasing load but the pumping energy remains constant. Modern chillers with microprocessor control, using sophisticated algorithms are now able to safely operate with variable flow rates which their older predecessors were not. This permits the use of variable flow rate circuits achieved with the use of 2-port control valves. This is achieved by using 2-port control valves at the emitters so that the varying flow rate through the emitter is reflected in the main hydraulic circuits. Back at the circulating pump, by sensing the rise in differential pressure of the circuit as the control valves close, the pump speed can be reduced automatically by the use of an inverter thus reducing pumping energy by the reciprocal of the flow rate reduction cubed. For example, if the flow rate halves then the pumping energy will reduce to 12.5% (1/2^3) of full load. Therefore, not only are variable volume flow designs lower in capital cost to install, they can also realise significant annual savings in pumping costs. Of course, variable flow systems are not a panacea and can have their difficulties if not designed correctly but when correctly implemented can have significant cost benefits over traditional constant flow systems.