Why close control is evolving to precision air conditioning

Why close control is evolving to precision air conditioning

Airedale International Air Conditioning project coordinator Adam Yarrington looks at why conventional close control is no longer ‘close’ enough and how latest technology precision air conditioning offers more value and more precision for a great deal less energy.

Few of us would willingly revert back to an on/off switch to operate our domestic central heating system. The ability to vary and preset heating systems through thermostatic control not only suits our lifestyles, it significantly reduces energy bills.

Traditionally ‘close control’ has featured ‘on/off’ temperature control with little regard to energy consumption. As the temperature in the room rose, the unit switched to cooling but overcooled beyond the room setpoint. In doing so, it removed water, as cooler air is less able to hold moisture. The moisture then had to be replaced, in effect by boiling the water to create humidification. Once the unit switched off the cooling, the room temperature gradually rose but then overheated and the cycle repeated itself, wasting power in unnecessary heating, cooling and humidification.

30% increase in system efficiency
Precision air conditioning is being driven towards efficiency with new technology and by the demands of today’s 24/7 critical environments. 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.

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. Thermal loads in data centres are continually fluctuating and many servers are under-utilised. Intelligent precision air conditioning 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 air conditioning 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 utilising 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.

Efficient de-humidification
If we precisely control temperature, we can precisely control humidity. The balance between temperature and relative humidity is intrinsically-linked. Change one and it changes the other.

With the intelligence and variable adjustments possible with precision air conditioning, rehumidification - putting humidity back into the room - 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 air conditioning, variable de-humidification 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, reducing the fan speed saves substantial energy especially using variable performance EC fans which respond seamlessly to changes in load. Typically a 50% drop in air volume will result in an 83% reduction in fan power input.

EC fans facilitate constant air volume when faced with variable system resistance such as dirty filters or ductwork extensions. The fan automatically senses the pressure change and adjusts fan speed accordingly to maintain operating air volume. Without constant air volume, system performance drops off.

Constant pressure control
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 air conditioning 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 air conditioning, 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.

Precision air conditioning technology 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, 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 PUE1 and take control of reducing energy costs.

1 PUE - Power Usage Effectiveness

About Airedale precision air conditioning
Airedale’s most advanced precision air conditioning solution is the SmartCool, designed for high efficiency and resilience. It offers a choice of seven system configurations in DX R410A and/or chilled water and in single/dual circuit or in dual cool configurations with free-cooling variant. The SmartCool offers up to six stages of cooling, a ‘blow through’ design for enhanced air flow and a host of options including EC fans, inverter compressors and variable air volume.

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