Airedale sets new levels of energy efficiency for air-cooled chillers

Published:  08 April, 2008

Airedale International Air Conditioning has taken a major step forward in energy-efficient chilled-water air conditioning with the development of a range of chillers based on the Danfoss Turbocor compressor combined with a series of energy-efficiency features engineered by Airedale.

A TurboChill air-cooled chiller with a cooling capacity of 550 kW has a seasonal energy efficiency rating of over 5.3 — 50% higher than the 3.56 of an Airedale OptiChill chiller with a screw compressor and a cooling capacity of 538 kW. Put simply, that means the annual energy consumption is reduced by a third.

So large is this increase in efficiency that the £8838 higher cost of a 540 kW TurboChill chiller compared with a 538 kW OptiChill is dwarfed by the £21 554 reduction in annual energy costs (based on 9 p/kWh). UK sales director Paul Oliver says that the payback is less than five months.

Now read on.

The Turbocor compressor was first conceived in 1993 as a small centrifugal compressor with a cooling capacity in the range 200 to 300 kW. Other design criteria included being direct driven and not using a lubricant.

It was not until 1999 that stage-one manufacturing commenced in Canada and 2004 before Danfoss and Turbocor formed a 50/50 joint venture that led to this compressor entering volume production.

Airedale was quick to see the potential benefits of this truly revolutionary compressor and in 2005 began a 3-year development programme that has recently culminated in the launch of the TurboChill range of air-cooled chillers with cooling capacities of up to 1100 kW.

Before explaining how Airedale has incorporated the Turbocor compressor into the TurboChill range of chillers, which use R134a, a few words about the compressor itself. The need for lubricants has been eliminated by using magnetic bearings for the shaft. The motor itself is a high-speed synchronous permanent-magnet type and drives the impeller directly. It weighs just 120 kg, compared with about 480 kg for the screw compressor used in Airedale’s OptiChill range, and is also less than a third of the size.

The Turbocor compressor is also very quiet — 70 dB(A) at 1.5 m — and has a fully integrated control system to control the electromagnets that trim the support of the shaft and to provide inverter control of the speed up to 48 000 rev/min. The behaviour of a centrifugal compressor is similar to a fan, and a 20% reduction in shaft speed reduces the power input to 51%.

Because this compressor does not require any oil for lubrication, there is no oil to contaminate the refrigeration system if it should burn out. However, there has not yet been a single case of burn out of 3000 compressors in Europe with a accumulated running time of two million hours.

With oil contamination not being an issue, Airedale has designed the TurboChill with a single refrigeration circuit connected to up to four compressors. Capacity is controlled by a combination of turning off compressors and varying their speed. The single refrigeration circuit makes the entire heat-exchange surface available at all times, increasing efficiency at part load and improving the ESEER (European Seasonal Energy Efficiency Rating) by 8 to 10%.

Chiller Airedale
This model of Airedale’s TurboChill air-cooled water chiller has two compressors (in acoustic enclosures) linked by a single refrigeration circuit to a single-pass evaporator. The shaft of the Turbocor compressor runs in magnetic bearings and does not require oil. To the right is a sickle-bladed condenser fan with ECDC motor that can have its speed varied to control the condensing temperature to maximise EER.

The single-pass evaporator is said to offer the performance of a flooded evaporator, but with 30% less refrigerant. Evaporating temperatures are above 5°C for 7/12°C chilled-water systems, and improved superheat stability enables a lower setpoint to be used.

The controllability of the Turbocor compressor enables it to exploit the benefits of electronic expansion valves (EEVs) to the full to reduce energy input and increase cooling output. EEVs offer greater system efficiency due to reduced head pressure at part load.

An EEV enables a screw compressor using R134a to operate down to about 4 bar before oil circulation becomes a problem.

Turbocor chillers do not use oil and can be operated down to a compression ration of 1.5 to double the EER at part load. Indeed, an EEV enables the condensing temperature to be reduced down to 20°C, compared with 33°C for a screw compressor.

The next stage of maximising the efficiency of a TurboChill chiller is to use electronically commutated DC motors to drive the condenser fans and provide speed control.

Increasing the speed of the condenser fans reduces the head pressure on the compressor, resulting it drawing less power — but at the expense of the fans drawing more power. The trick is to minimise the total power consumed by the fans and the compressor — and the TurboChill decides for itself what is the best head setting.

The capabilities of the Turbocor compressor and the design philosophy of the TurboChill have enabled Airedale to rationalise manufacture.

Depending on the capacity required, two, three or four compressors are used. Only six different coils are used with eight to 20 fans to provide up to 210 permutations of cooling capacity from 500 to 1100 kW — making capacity data tables irrelevant.

And, finally, every permutation of TurboChill chiller qualifies for Enhanced Capital Allowances.

April 08 70

For more information on this story, click here:April 08, 71

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