ReChill® upgrade to York Chillers
The Chillers serving Fujitsu offices in Stevenage are grouped in to two sets of two York chillers originally rated at 580 kW. However, the chiller had proven very troublesome due to the apparent lack of load, leading to the chillers working continuously upon their minimum load setting.
The chillers concerned had suffered from ongoing trips and failures, and whereas numerous call outs had been responded to, nothing was resolving the ongoing situation of non reliability.
Further assessment also showed that forever running the compressors at minimum load was causing secondary problems of excessive compressor slide valve wear, adding to the issues experienced and that the compressors were operating at their least efficient. This is a design limitation of slide valve load controlled compressors - the more they operate unloaded the less efficient their output (kW Output as Cooling duty : kW Input as electrical power).
The upgrade re-design had to resolve the problem of non reliability and a considerable secondary benefit would be the element of payback that might be achieved from improved operating efficiency. Hence the brief we offered was to assign now established Inverter Screw compressors along with new controls and whilst retaining the efficient R134a gas already assigned.
The alternative proposal for the clients to consider was simply to replace the chillers, which at just 6 or so years old was hardly an attractive prospect.
The project has also minimised site inconvenience and there was nil disruption to provision of chilled water services throughout the project, conducted between May and July of 2014.
As is expected practice, 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, and enabling them to promote the re-cycle by re-use principle.
Screw Compressors & Load Control by Variable Frequency Drive Inverters
Re-Chill® has proven modern chiller efficiency can be achieved on most older chiller frames 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 progressively 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 motor control, but this is on fully designed from scratch equipment. The Re-Chill® challenge was to apply this to an elderly Chiller and achieve the same effective gains…
- Smoother start / reduced in rush current & power spikes – starts on 7 amps
- Effective speed control provides variable load from 60 - 140% compressor standard speed of 50 hz (30 - 70 hz)
- Start / Stop cycles drastically reduced
- VFD Speed control can over-speed compressor to 140% of standard
- Therefore a smaller compressor is selected, providing both cost and load turn down benefits
- Duty for Duty the turn down ratio is ~ 40% system load, or 20% chiller load
- Reduced rotor tip blow by at lower speeds further improves volumetric efficiency
- 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.
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 screw 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.
The upgrade to Inverter Screw compressors 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.
Further peripheral savings were made by combining the project with new Fan speed control inverters and by integrating the chillers control Target Reset feature to work with the existing BMS system, allowing the chilled water temperature to be varied with the external ambient, and consequent load demand, which further improves the energy efficiency, by optimising chilled load to the environmental conditions hour by hour, day by day.
© Trevor Dann - August 2015