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Natural remedy

Ammonia was the refrigerant of choice to replace R22 at Homerton Hospital in London. RAC finds out why

J&E Hall has designed, built and installed an ammonia-based system at Homerton University Hospital in Hackney, east London, where it cools the operating theatres, the maternity and the intensive care units - so safety is a high priority.

Stand-out features are U-tube suction separators fitted to the chillers, which reduce the refrigerant charge by a third, and a safety shutdown programme ensuring that any leaks are contained within the system. The two special Hall Aquachill units with 0.5 MW of cooling capacity each are employed to cool water from a temperature of 12 deg C to 6 deg C.

Mark Colley, sales and marketing manager for J&E Hall Special Products, says the hospital authorities had at first been hesitant to consider a natural refrigerant

option to replace an existing Hall cooling system, running on R22. But they were won over once they had been told about the efficiency of the equipment and safety measures in place to support the system.

Mr Colley explains: “The suction separators manufactured by Alfa Laval were on show at Chillventa last year. The design is in common use in Germany, where controls on ammonia-based systems are tighter than in the UK. They proved to be an ideal choice for the installation at Homerton Hospital.”

The system took many months to design and build by the Hall team. On top of this, an existing 340 kW cooling system from the company, running on R22, had to be decommissioned. More than 300 kg of R22 were removed from the old system and replaced with a little over 50 kg of ammonia in the new system - 15 per cent of the refrigerant previously required. Hall is sending off the R22 for possible reclaimation.

The two Aquachill units give the hospital authorities a 50 per cent increase in cooling capacity, although they had to fit into the same small plant room, with the existing system still meeting the demands of a busy hospital.

Mr Colley explains the appeal of the suction separators: “Applied to a plate evaporator, the advantages of a thermosyphon system are retained but with a volume of refrigerant in the chiller similar to a DX unit. A water-cooled condenser - another plate heat exchanger - is coupled to the dry-air coolers.  It’s like a dry cooling tower, keeping the ammonia refrigerant within the chiller pack rather than using a remote condenser. Again, this makes the ammonia charge smaller and easier to contain. What’s more the entire system is enclosed in acoustic panels, reducing noise and improving safety.”

The system was designed by Hall Special Products in Dartford, Kent, and installed by the company’s regional contracting division. Applications engineer Steve Gowing co-ordinated the installation. He says that if there should ever be a leak, not a drop of ammonia will escape into the atmosphere, thanks to a scrubber behind the system.

“There are extract fans to remove the motor heat and an ammonia detection system near the exhaust fan,” he says. “So any air that passes to the outside has to go over the ammonia detector. Even if the smallest amount of ammonia escapes, the system will shut the extract fans down and start the air scrubber.

“Once the air scrubber starts, a water pump produces a fine spray to wash the ammonia from the air. Passing through the water spray the ammonia is absorbed into the water - ammonia and water have a great affinity for each other. The air scrubber water tank has been sized to accept the entire ammonia charge of the system. Each chiller is encased in a sealed acoustic enclosure so, even if the whole system ammonia charge does leak, the air scrubber can absorb it all in the water.”

There are no long pipe runs carrying ammonia to and from the roof, he adds. “Using a water-cooled condenser means the complete chiller is contained in the enclosure and all we have to do is pump water from the condenser up to the dry air cooler on the roof.  If we had an air-cooled condenser, we would have to pump ammonia to the roof and back to the chiller.  The condensers are 80 m away, so we would have had 80 m of liquid pipe containing ammonia coming back from the roof, which would increase the refrigerant charge considerably.”

Hall’s natural refrigerant system is suitable for large public buildings and sports stadiums. There are plans to extend the system to other parts of the hospital.  The key to success at Homerton is down to thinking ahead. Mr Gowing says: “When installing a system like this in such a small space, while an existing system continues to operate, needs careful planning.  The Hall design team did a great job on this.”