Evaporative Cooling Systems
Most modern manufacturing facilities require cooling for either the comfort of the occupants or to protect the processes or equipment they contain. There are three methods which can be employed to cool these buildings; mechanical ventilation, refrigeration-based air conditioning or evaporative cooling.
Evaporative cooling is fast becoming a popular alternative to air conditioning and mechanical ventilation, as it is able to meet the demands of facility cooling without the associated high capital and running costs.
Principle of Operation
Evaporative cooling uses a very simple method of cooling air without using refrigerants. The system relies on a widely recognised thermodynamic principle known as adiabatic cooling, employing water’s large enthalpy of vaporization. In fact, water has one of the highest known enthalpy of vaporization values of any common substance.
The temperature of dry air can be dropped significantly through the phase transition of liquid water to water vapor (evaporation), which can cool air using much less energy than refrigeration.The evaporation of water requires the taking of latent heat from the ambient atmosphere; the consequence of this is that the ambient air decreases in temperature.
The energy needed to evaporate the water is taken from the air in the form of sensible heat, which affects the temperature of the air, and converted into latent heat, the energy present in the water vapor component of the air, whilst the air remains at a constant enthalpy value. This conversion of sensible heat to latent heat is known as an adiabatic process because it occurs at a constant enthalpy value. Evaporative cooling therefore causes a drop in the temperature of air proportional to the sensible heat drop and an increase in humidity proportional to the latent heat gain.
External air is drawn through a mat, which is kept wet by a constant supply of water. A stainless steel sump at the base of the unit is maintained full by a valve and water is re-circulated through the mats using a pump and distribution system. As water passes over the mats, a proportion is evaporated into the air stream thus reducing its temperature, the amount of cooling being a function of the external air temperature and its relative humidity. This cooled air is then distributed within the area to be treated via a ductwork system.
A valve arrangement on the drain dumps the contents of the sump when the unit is no longer in operation, reducing the risk of bacterial growth. This however, does not eliminate the need for cleaning of the sump at the beginning and end of the cooling season. Additionally, fitting the evaporative cooling unit with an ultraviolet water treatment lamp will sterilise the water as it is circulated and reduces any possible bacteria growth.
The electrical control panel supplied controls the main functions of the unit. A selection switch on the front of the panel allows the unit to be operated in either ventilation only or cooling modes. A thermostat and/or humidistat can be connected if required to control the fan and water pump independently. This enables high and low temperature and humidity settings to be dictated.
Ventilation systems can provide comfort cooling for most of the year. During prolonged periods of high temperatures they are unable to maintain internal temperatures below 25°C. Refrigeration based systems are effective but can be expensive to operate.
Evaporative cooling, which can produce air consistently below 22°C in the UK climate, provides an intermediate solution which consumes only a fraction of the electricity. In fact, they can typically consume less than 10% of the electricity compared to the equivalent rated refrigeration based cooling system. In the UK, humidity is relatively low when the temperature is high, therefore a drop of over 10°C – 12°C during peak temperatures can be achieved.
When the external temperature is below the set-point of the unit an evaporative cooling system will operate as a mechanical ventilation system, supplying large fresh air volumes into the building replenishing stale air at a high rate. In addition, the simplicity of an evaporative cooling unit design results in low annual maintenance costs and high component durability levels.
Evaporative cooling systems are a proven solution within the following industries:
- Chemical & Pharmaceutical
- data centres
- modern offices