Since switching to a new ammonia-based cooling system based around Alfa Laval heat exchangers, the Homerton University Hospital has increased cooling capacity by 50 per cent.
At the same time, the UK hospital has significantly reduced refrigerant charge and reduced its overall carbon footprint.
Initially, the hospital was wary about using natural refrigerants in its cooling system. But the impressive results in efficiency, reduced greenhouse gases and enhanced safety features convinced the hospital staff to go with ammonia.
The new system has been designed and installed by J&E Hall and comprises two J&E Hall Aquachill refrigeration modules based around Alfa Laval heat exchangers.
Each of the modules provides 0.5MW of cooling capacity. Built on a purpose-designed skid complete with all controls, each Aquachill module includes an AlfaNova all-stainless steel heat exchanger, two twin frame M10BW plate heat exchangers for evaporating and condensing, and a U-Turn liquid separator.
Linked to two roof-mounted 612 kW dry coolers, the system provides cooling for vital areas of the hospital such as the operating theatres, maternity unit and intensive care units.
Thanks to the modular design and the use of U-Turn separators, the refrigerant charge is 33 per cent less than in a conventional ammonia system. It not only delivers highly efficient cooling but also eliminates greenhouse gas emissions.
A failsafe shutdown program ensures that any leaks are contained within the system - a vital feature given the sensitive nature of the location. J&E Hall applications engineer, Steve Gowing, said the safety features built into the system will ensure if there is a leak.
Not a drop of ammonia will escape into the atmosphere. “With the transition, more than 300kg of R22 has been replaced with just over 50kg ammonia,” he said.
“Despite the huge reduction in refrigerant, the hospital has achieved a 50 per cent increase in cooling capacity.”
As the system was being installed and the old 340 kW cooling system was being decommissioned, the new system had to be sufficiently compact to fit into the same small plant room as the old system for the time it remained in operation.
Central to the compact nature of the Aquachill units are the U-Turn liquid separators, specifically designed to fit within the frame of the heat exchanger/evaporator.
J&E Hall engineering director, Dave Ball, said this gives them a significant advantage over conventional surge drums, which are generally much larger and heavier.
“Used in conjunction with the plate evaporator, it retains the advantages of a thermosyphon system but with a much smaller volume of refrigerant in the chiller, similar to a DX unit,” Ball said.
“It’s like a dry cooling tower, enabling us to keep 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 accoustic panels, reducing noise and improving safety.”
The U-Turn liquid separator is especially designed for use with plate heat exchangers in ammonia applications. It is lightweight and compact, making it easy to install. It can cover ammonia capacities from 200 to 1,400 kW at 0° C evaporation temperature and from 50 to 500 kW at -40°C evaporation temperature.
U-Turn is designed to take advantage of the very best of Alfa Laval plate heat exchanger technology. This includes efficient separation by means of agglomeration, gravity, centrifugal forces and surface tension.
Low internal pressure losses mean good part load performance.