Meaner Cleaner Cooling for Greenwich Mean Time

Re-Chill® system upgrade options provided by ThermOzone, have been taken to a new phase of development to save energy and provide a more effective cooling solution for the World renowned home of Longitude and Time itself.

The National Maritime Museum in Greenwich, setting of the Prime Meridian, and home to the famous John Harrison Clocks that first pinned down the concept of ‘Fixed Longitude’ for marine navigation.

greenwich dm

In order to preserve the precious relics in the main Museum complex a closely controlled environment is essential.

Several years ago the Museum chose to install two water chillers manufactured at the now defunct York International - Basildon factory. Although appointed on HFC R134a, still the refrigerant of first choice for most newer water chillers, these specific chillers from 1998 have provided reasonable service, but with their 20th Century reciprocating compressors, the chillers were seen as approaching their life-point and were far from energy efficient.

However, the chillers have been well maintained and remained in sound order. Despite the 15 years since installation, the technology of the major chiller assemblies, primarily a shell and tube evaporator and remote air-cooled condenser assembly, has not seen that much improvement in operating efficiency and current requirements to control price has seen reliability & longevity worsen.

Therefore the proposal to replace the chillers entirely would have meant a substantial local assembly chiller, into the enclosed plantroom with much new, but otherwise un-necessary metalwork assemblies, and possibly for equipment with a lesser lifespan even than the Upgrade option.

Instead ReChill® offered the benefits of a new Chiller from a technology and energy perspective, but with certain major refrigeration assemblies and chassis retained, providing considerable cost savings for the clients.

The upgrade re-design had to provide an effective payback and ongoing reliability and efficiency. The project also required to be cost effective from a Capital outlay perspective.

Screw Compressors & Load Control by Variable Frequency Drive Inverters

ReChill® has proven modern chiller efficiency can be achieved on most older chiller frames simply using robust Screw compressors and modern Controls. However, screw compressors, albeit far more efficient on these applications than piston compressors, themselves have a load efficiency weakness, whereby at partial load the internal load control ‘Slide Valve’ reduces energy efficiency increasingly below 100% load (displacement volume), with efficiency falling rapidly below 60% slide valve / swept volume at low external load. So effective seasonal load control to satisfy modern demands for an effective SEER / ESEER (efficiency rating) is a crucial area where innovation is very necessary.

New manufacturers have already shown Inverter technology can provide variable speed control as an effective mode of compressor control, but this is on fully designed from scratch equipment. The ReChill® challenge was to apply this to an elderly Chiller and achieve the same effective gains...

1: Smoother start / reduced in rush current & power spikes – starts on 7 amps
2: Effective speed control provides variable load from 60 - 140% compressor standard speed of 50 hz (30-70hz)
3: Start / Stop cycles drastically reduced
4: VFD Speed control can over-speed compressor to 140% of standard
5: Therefore a smaller compressor is selected, providing both cost and load turn down benefits
6: Duty for Duty the turn down achieved is ~ 40% system load, or 20% chiller load
7: Reduced rotor tip blow by at lower operating speed further improves volumetric efficiency
8: COP / EER measured above 5 at low load operation and > 4 at full load

Proven energy savings of Screw compressors replacing Piston compressors provides an energy saving closely linked to the lesser pumping efficiency due to cylinder & valve losses of the piston compressor compared with the screw equivalent displacement – e.g. circa 25 – 35%. However, at lower load settings (for most chillers ~ 70% of their annual operating cycle), the Screw compressor is not optimised, with the slide valve modulating the load below its optimum efficiency band.

With VFD Inverter control the Slide Valve is held at full load position, optimising the screw compressor at its most efficient. By simply slowing it to match the chilled water cooling demand (the load), the specific efficiency increases, because with slower internal gas flow-rates, internal pumping losses are reduced, with less blow-by of compressed gas across the rotor tips. Maintaining the oil lip sealing is simpler as the slower motion is not dragging the oil as much as at higher speeds, although the minimum speed is limited by the requirement for sufficient low – high pressure differential to maintain sufficient oil flow to the screw mechanism and bearings.

Preliminary measurements indicate the Inverter control will improve part load operating efficiency by as much as a further 30%, and overall it is anticipated the running energy saving will be above 50% all round compared with the original piston compressors. A considerable benefit is the very low start current from the ‘Soft Starter’ feature of the Inverter.

The early May Bank Holiday week of 2013 saw the first of the Chillers at the Museum started, and the early energy results are quite astounding, indeed the initial starting currents were so low special techniques were required to achieve excitation of the Current Transformers measuring the input energy. The chiller as a whole is quieter than the adjacent chilled water pumps, indeed dedicated Run lights are provided to show maintenance staff which individual compressors are running.

Associated Works

The upgrade to Screws requires dedicated Load Control technology, achieved from the Magnum Controller, which fully supports Variable Frequency Drive Inverter speed control. The Magnum also provides ancillary chiller control functions – Condenser head pressure control, also via VF Inverter Drives; Electronic Expansion Valve control; System rotation and comprehensive system fault monitoring.

A Further Energy Bonus

It was found during project preliminaries the original condensers had skimped on the Fan Speed control, using a single inverter to drive two sets of fans. Hardly energy efficient - both sets of fans running when at the part load setting of just one compressor (i.e. ~70% of the operating time). It is estimated this extra finding and the local decision to add a second VFD here also will add a further unexpected saving of some 5-8% overall chilled water operating energy costs.

© Trevor Dann - May 2013