York HVAC Catalogue 2018 EN 2019 EN | Page 80

HVAC Fundamentals Water Systems Chilled water systems An optimisation of the system price, function and efficiency must consider all components and their interaction. It starts with the load calculation. A floating temperature setpoint in the comfort range area will save energy and reduce operating costs. Capital costs can be reduced by balancing the selection of Chillers, Air Handling Units, Ductwork sizes, etc. It is important to determine the optimum operating point that balances the selection of the Chiller leaving water temperature and the Air Handling Unit cooling coil. A temperature rise of 1°C in water temperature yields approximately 3% more capacity for the Chiller and reduces the absorbtion input power typically by 1.5%. However the coil capacity reduces with temperature rise and requires larger heat exchange surfaces (more rows and/or a lower fin spacing). Air conditioning system designs normally use supply chilled water temperatures of 5°C to 8°C. Some systems, such as chilled ceilings or beams, may use higher temperatures up to 14°C or 15°C. If leaving temperatures less than 4.5°C are requires brine solutions are used to prevent freezing. This is specially the case with Ice Storage Systems that can have temperatures as low as minus 7°C. The cooling capacity of a Chiller increases with rising leaving chilled temperatures. A temperature difference, between flow and return, of 5°C to 8°C is normal. The water flow volume is dependent on the cooling capacity and chilled water temperature difference in the following formula: If the leaving water temperature of the Chiller is raised it is possible that one Chiller size smaller can be selected. COOLING CAPACITY (kW) Water Flow Volume = (Litres per Second) Density (kg/m 3 ) x Specific Heat (kJ/kg°C) x Temperature Difference °C x1000 The capital cost for the larger coil is comparatively small and the cost savings of a smaller Chiller can be considerable. The resulting water flow must be checked agains the flow limitations of the Chiller. This can be found in the “Limitations Table” for each type of Chiller or heat pump (data is not in this catalogue). Increasing the leaving chilled water temperature will also increase the air temperature leaving the Air Handling Unit coil and this may in turn decrease the supply and return air temperature difference. A small temperature difference achieves a low MEAN WATER TEMPERATURE which will generally allow the selection of smaller cooling coils in Air Handling Units and Fan Coil Units etc. Conversely water flow volume will be high resulting in a larger circulating pump and increased pressure drop through the Chiller and coiling coils and a consequent increase in operating costs. The Air Volume is determined by the following formula: Air Volume m 3 /s = HEAT GAIN (kW) Density (kg/m 3 ) x Specific Heat (kJ/kg°C) x Temperature Difference °C A smaller air temperature difference will increase the air volume and therefore the duct sizes and resultant cost of the ductwork. It is therefore important to consider the total impact on all the components of the air conditioning system. Lower supply air temperatures will reduce the size of both ductwork and Air Handling Units and specially designed air diffusers can be used to ensure that the lower supply air temperatures have no adverse effect on the building occupants. The pressure drop varies as the square of the flow and is defined in the following formula: H2/H1 = (W2/W1) 2 H1 = Pressure Drop kPa at final condition H2 = Pressure Drop kPa at original condition W2 = Flow rate L/s at final condition W1 = Flow rate L/s at original condition Selecting the optimum temperature difference is therefore a compromise between operating costs and equipment size and the capital cost of such equipment. Primary chilled water temperature differences are normally between 5°C and 6°C. Generally a minimum system flow volume will provide the least expensive system in both capital and operating costs. Piping system design On larger air conditioning systems it is generally recommended that “Reverse Return” piping arrangements are used to ensure balanced flow rates. An Air Conditioning system in a building comprises a variety of components, such as Chillers, Air Handling Units, Diffusers, Ductwork, Pipework, Controls, Electrical Wiring, etc. Chiller CHILLER CAPACITY CURVES OPERATING POINTS 4 Rows 6 Rows 8 Rows COIL CAPACITY CURVES Water temperature or refrigerant evaporation temperature 80