THROUGH simultaneous variable control of temperature, humidity, fan speed and capacity, the latest precision air conditioning can typically provide operators with a 30% increase in system efficiency compared with previous generation close control systems, according to Airedale project coordinator Adam Yarrington.
'Inefficient and slow to respond, conventional 'close' control is no longer 'close' enough for premium data centres and other critical environments where sensitive computer equipment operates best within tight tolerances of temperature and humidity,' he says.

Thermal loads in data centres are continually fluctuating and many servers are under-utilised. Adam Yarrington maintains that intelligent precision ac not only precisely controls temperature and humidity; it reacts to changes in heat density and humidity resulting from the fluctuating IT load.

Refrigerant and chilled water-based products involve moving liquids around a system, expanding and condensing it to change temperature. To precisely control temperature and humidity, precision ac can vary refrigerant or chilled water flow and simultaneously, fan speed.
Precise and variable cooling

In precisely controlling temperature, the flow of refrigerant in DX units can be varied by using inverter compressors or staged cooling and/or special throttling devices. Staged cooling, using tandem scroll compressors across multiple refrigeration circuits, gives up to six stages of cooling compared with only one or two stages of cooling with conventional close control. By using the maximum coil area within the space, staged cooling gives high levels of control and increases part load efficiency.

Latest technology 30-90Hz inverter-driven scroll compressors allow variable refrigerant flow and fully modulated cooling from 25-100%. Room temperature is kept tightly near setpoint at all times by continuous load matching operation. Inverter driven compressors save energy when operating at part load especially when combined with variable performance EC fans.

'In chilled water units we can vary the chilled water valve position and the fan speed to give precise and variable cooling. If this is combined with a variable flow water system, pump power can be minimised to further enhance the system efficiency,' he adds.
'If we precisely control temperature, we can precisely control humidity. With the intelligence and variable adjustments possible with precision ac, rehumidification is rarely necessary. As the room conditions are more precisely controlled, the time and energy needed to re-humidify is greatly reduced, saving precious power.

'In latest technology DX precision ac, variable dehumidification is highly efficient. If we put a stage of cooling on and reduce fan speed, the air spends more time on the coil; with an evaporating temperature below the dew point, the air is then forced to shed moisture. Significantly less mechanical cooling is used compared with conventional de-humidification. Conventional close control units have fixed fan speed, so instead of slowing down the air, mechanical cooling is applied to make air as cold as possible.'

Besides assisting with temperature and humidity control, Adam points out that reducing the fan speed saves substantial energy especially using variable performance EC fans. Typically a 50% drop in air volume will result in an 83% reduction in fan power input.

Constant pressure control

He maintains that a better option still, for ensuring even air flow to server racks, is to control air pressures within a space such as a floor void or a duct, using an EC fan modulated to maintain a given setpoint.
'Ideal for hot and cold aisle configurations, constant pressure control adapts to fluctuating server loads and changes within the room such as the addition of extra floor tiles. Any fluctuations in resistance accumulated over the life of the data centre will be compensated for by constant pressure. The varying air flows are matched by the precision ac unit.

'Sensible heat ratio control with minimum re-heat ensures 80% less power is absorbed during the de-humidification cycle. Hot gas re-heat offers an energy efficient way of providing re-heat capability during the de-humidification process by utilising heat energy that would normally be rejected outside by the condenser. No further energy is needed over the standard power required for the refrigeration circuit. A 100kW unit with multi-stage cooling and hot gas re-heat, presents an 88% saving in input power compared to a conventional method of control using electric heat. With latest technology precision ac, only the first stage of cooling, coupled with reduced fan speed, is required to perform the de-humidification mode.'

Thyristor controlled electric heating offers fully modulated control of heating and reheating. 'It pulses to modulate heating and is ideal for laboratories and standards rooms where precise control is required.'

Ian points out that precision ac can now include free-cooling as an option and gives even greater resilience when combined with a dual cool system. The latter can feature both DX and chilled water within the same case, supporting 2N redundancy specifications and providing automatic change over and duty share.
'Looking forward,' he says 'New opportunities to develop precision air conditioning through variable control technology will ensure power is used only when and where it is needed, enabling operators to lower their PUE (power usage effectiveness) and take control of reducing energy costs.'