NuChill® upgrade to York Chillers
Developing from the well established ReChill® concept, it was apparent that certain projects required all new chillers, rather than partial upgrades leaving some original sub-assemblies in service. Clearly this might compromise the overall life expectancy of a ReChill® project. However, all of the individual techniques of ReChill®, if applied together, do effectively provide a new functioning chiller- a ‘Bespoke Built’ chiller. This is NuChill®.
NuChill® offers Clients a bespoke option based upon the ReChill® concept, but importantly we can provide a new chiller within an existing chassis. Invariably the chassis of even the oldest chillers are perfectly good, perhaps needing a facelift, but they provide a superb platform into which to install the new (re)designed chiller. NuChill® also offers full bespoke chillers into difficult locations where an off the shelf ‘box’ will simply not fit.
Inmarsat is a World renowned communications company specialising in Satellite based telecommunications. This project was for the water chillers providing cooling services to their prestigious offices adjacent to City Road / Old Street Roundabout in the City of London.
Air-conditioning cooling source for the building has for the past 30 years been provided by three elderly York water chillers situated in a roof top plant area. These were installed during the building’s original construction in ~1988. Now reaching beyond their original 20 year life point the chillers had become less reliable, and were costly to run being of a much lower efficiency standard than currently expected. Following the demis of HCFC R22 the chillers have been effectively unserviceable and hence on borrowed time since 2014.
Replacement created a major headache for the Inmarsat - heavy Chillers of ~650 kW cooling capacity with access only from a very busy Red Route traffic junction. Craneage just to renew mechanical plant could have easily doubled the base cost of replacement chillers. However, ReChill® or NuChill® offered a ‘through the building solution’.
ThermOzone have been supporting these York style chillers for many years so were familiar with the models concerned. Thus it was obvious to ourselves these chillers could be completely renewed in this way.
This also presented opportunity to improve both energy efficiency and resiliency.
Inverter driven and controlled Screw compressors; Electronic expansion valves; EC Fans; High efficiency condensers; Compact and high efficiency evaporator.
The original capacity was maintained but the re-design allowed us to reset this to a higher performance point. These 650kW chillers were originally sized for ~28oC ambient, a temperature routinely exceeded in the heat layer bubble that sits over the built up area of London on warmer Summer days, to a more capable 35oC. Indeed it is forecast the chillers will remain serviceable to an air on temperature in excess of 40oC, albeit with some loss of capacity above 35oC. This was achieved by selecting higher capacity compressors and increasing heat rejection and hence tube volume of the condensers, plus maximising the airflow capacity using new EC fans atop the condensers.
As part of the NuChill® design ThermOzone conceived replaced all functioning assemblies using selected high quality components and sub-assemblies throughout. The retained chassis not only removed the need to take the roof off, but provided a significant element of recycling by re-use.
Crucially for the Clients benefit the entire renewal process was undertaken via the pedestrian access route, and a single crane lift using a small crane unit from neighbouring yard, minimising local disturbance. By undertaking the least reliable chiller first, service was maintained throughout the project, conducted between August 2017 and July 2018. The first year’s full operation is allowing for seasonal adjustment during the soft landing phase.
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 variable load from 60 - 140% compressor standard speed
- Sizing is made at the duty high point - therefore a smaller compressor is selected
- This provides both cost and load turn-down ratio benefits
- Start / Stop cycles drastically reduced
- VFD Speed control can over-speed compressor to 140% of standard
- 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 above 4.5 at standard 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. ~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 controlling its speed 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 high to low 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 ~44% allowing for use of R134a all round compared with the original compressors with R407C. A considerable benefit is the very low start current from the ‘Soft Starter’ feature of the Inverter.
The upgrade to Inverter controlled 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.
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.
Project 1 - Chiller 1 Autumn 2017 - ReChill® (as NuChill® spec save Condesners which were retained)
Project 2 - Spring & Summer 2018 - NuChill® including new condensers
© Trevor Dann - January 2019
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