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