Data cooling for free

They installed a 'thermosyphon cooling system' which uses the same chiller components as other vapour compression refrigeration system, but with a novel twist. Low cost water cooling comes from bypassing the compressor – the chiller's main energy consumer.

This is possible when ambient temperature is cooler than the water to be chilled. There are three requirements for maximum free cooling. First, high water temperatures, which raise the difference between water and ambient temperature, so improving heat transfer.

Second, a constant load profile. Because the system is slow to switch between mechanical and thermosyphon mode, it struggles to cope with fluctuating loads. The third requirement is for multiple chillers, which allow the flexibility to bring units on stream to maximise thermosyphon cooling.

Until 1998 all thermosyphon projects were for either computer data centres or paint cooling in the motor car industry, which both meet the described criteria. The British Airways data centre is significant because it is the first time ammonia has been used as the primary refrigerant in this system of chillers.

The specification for the chilled water system called for air-cooled condensers and modular chillers supplying a cooling load of 2200 kW. This load is split between two separate chilled water circuits, each with three 550 kW chillers, giving a two run/one standby arrangement in each circuit.

British Airways stipulated that ammonia be used as a refrigerant, but was not prepared to sacrifice reliability or availability. The designers ensured that the system would be resilient by dividing services into independent 'strings', each with duplicate and standby plant. A clever arrangement of cross-connections also allowed a single string to support the full electrical load of the building and over 90% of the cooling load.

Using ammonia also had a major impact on housing the refrigeration system. There was no restriction on the ammonia charge, but the condensers needed to be carefully positioned in a specially constructed machinery room.

Modifying the chiller

The ammonia charge also forced designers to modify the packaged chiller. They had to make two changes: first to the evaporator – a shell and tube unit – and second to the oil management system.

Star Refrigeration was able to replace the normal cupro nickel tubing in the evaporator with an enhanced stainless steel tubing. The enhanced surface, made using a plasma-sprayed aluminium powder, produces tiny bubble nucleation sites on the tubing.

These nucleation sites provide seed bubbles which allow the refrigerant to boil with a reduced temperature difference. The enhanced surface significantly improves heat transfer in the tubing and can raise boiling coefficients up to 12 times higher than ordinary tubing.

The overall coefficient can be as much as four times greater than usual. This is important because it requires a larger temperature difference to initiate boiling than it does to sustain it. The difference between water and ambient temperature is what drives the boiling, so every degree trimmed from the difference boosts the free cooling available.

Oil management is a problem because, if oil is not returned to the compressor, it collects in the chiller, this can cut performance dramatically. Unlike the oil used in R22 systems, the synthetic polyalphaolefine required here can mix with the ammonia. This means that the distillation pot oil recovery system had to be modified.

Coefficient of performance

At the end of the day, the proof of the pudding is in the eating. The main incentive to choose a thermosyphon system is energy efficiency. Star and Justham Dunsdon's analysis of the system performance predicted an annual average coefficient of system performance (cosp) of 10·4 – incredibly efficient compared to conventional refrigeration systems.

To check this forecast, Star also carried out full load performance tests as part of the commissioning, using a boiler as a dummy load. With a dry bulb temperature of 8·58C, the mechanical mode test returned instantaneous cosp figures of 8·75.

With this temperature at 108C, and the plant set to run in thermosyphon mode, the tests returned instantaneous cosp figures of 9. Individual chillers operating in thermosyphon mode were achieving instantaneous cosp figures as high as 14.

On top of these performance coefficients, life cycle costing calculations over 20 years found the thermosyphon system to be 55% cheaper than the next best alternative. This equates to a £2.5 million saving at today's prices.

Unsurprisingly, the environmental benefit over conventional systems is also impressive – a reduction of CO₂ emissions of close to 15 000 tonnes over 20 years.