Displacement ventilation: better air quality, lower cost

Displacement ventilation works with high heat loads, occupancy levels and levels of contamination. And, says Richard Meskimmon (right), technical manager SP Coils Products, it’s energy efficient too
Displacement ventilation: better air quality, lower cost
PROVIDING good indoor air quality at an acceptable energy cost is a conundrum which has faced building services engineers for many years. Displacement ventilation is one approach which has proved successful, particularly where there are high heat loads, occupancy levels and levels of contamination as in theatres, concert halls and department stores.

The big advantage of displacement ventilation lies in the reduced air volumes that are required to condition the space effectively. Relatively low volumes of primary air are introduced at low level at a temperature of 3°C to 4°C below the design temperature of the space.

The stable layer of cool air rises when in contact with people and/or equipment, picking up heat and moisture. This warm air collects at roof level where it is extracted or cooled by a secondary passive cooling system (chilled panels and beams). Because of the lower air volumes being used, overall energy usage can be significantly lower while comfort is increased.

In order to prevent any condensation on secondary cooling equipment, the primary air must be cooled to a dewpoint of approximately 12°C and supplied at a temperature of 18°C to a space maintained at 21°C to 22°C. To achieve this supply condition, during the cooling season, the primary, ventilation air must be overcooled to reduce the dewpoint before being reheated to the supply temperature.

Based on a design outside condition of 28.0/20.0°C the primary air process is as follows, based on an air delivery of 1kg/s:

Outside air (28.0/20.0°C),

enthalpy = 57.3kJ/kg

Overcooled air (12.0/11.7°C),

enthalpy = 33.3kJ/kg

Cooling load = 24kW

Reheated air (18.0/14.0°C),

enthalpy = 39.3kJ/kg

Heating load = 6kW

At first sight the need to over- cool the supply air to remove moisture would appear to be very energy intensive. The energy required can, however, be dramatically reduced by the use of a heat pipe.

A heat pipe is an efficient thermal conductor, relying internally on the boiling and condensation of a working fluid to transfer heat along its length. By utilising the latent energy of the working fluid the internal temperature of the heat pipe remains constant while it pumps heat between areas of high and low temperature to which it is exposed externally.

Wrapping a heat pipe around the cooling coil in the ventilation ahu acts to pre-cool the air before it reaches the cooling coil and then adds this energy back into the air stream as reheat energy downstream of the cooling coil. The heat pipe effectively reduces or eliminates the load associated with overcooling while simultaneously reducing or eliminating the reheat load.

The loads associated with the introduction of the heat pipe are given here along with the savings that would be accrued based on the 1kg/s of primary air. The savings can simply be multiplied by the mass flow rate for other air quantities. The processes are demonstrated on the accompanying psychrometric chart.

Outside air (28.0/20.0°C),

enthalpy = 57.3kJ/kg

Precooled air (22.0/18.0°C),

enthalpy = 51.3kJ/kg

Overcooled air (12.0/11.7°C),

enthalpy = 33.3kJ/kg

Cooling load = 18kW, saving = 6kW

Reheated air after heat pipe (18.0/14.0°C), enthalpy = 39.3kJ/kg

Heating load = 0kW, saving 6kW

As the temperature of the outside air changes, the degree of energy transferred by the heat pipe varies. In order to control the temperature of the primary, supply air accurately, a trimmer heater battery should be included as the final control element of the outside air ahu.

The heat pipes are arranged to wrap-around the main cooling coil and add, typically, no more than 300mm to the overall length of the ahu. Heat pipes are made from copper tubes expanded into continuous aluminium fins in an identical manner to a conventional cooling coil. As such they are available in a continuous range of sizes to match the dimensions of the main cooling coil. Optional material specifications are available to again match the specification of the other cooling and heating coils in the system.

While this approach may seem novel to some, it has in fact been used successfully here in the UK, in the USA and in the more demanding climate of the Middle East for many years. There are many thousands of successful and proven heat pipe installations enhancing the indoor air quality of buildings, improving the comfort of their occupants while providing more efficient ways of energy usage.

SP Coil Products T: 0116 249 0044
1 July 2007


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