Climaveneta Chiller NuChill® Upgrade
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.
10 Old Burlington Street is a luxury high end suite of offices in a historic part of Mayfair. Air-conditioning cooling source for the building has for the past 15 years been provided by two Climaveneta water chillers situated in a roof top plantroom. These were installed during a major building refit and were positioned with the roof removed. Now reaching their life point the chillers had become less reliable since ~2010, and were costly to run being of a much lower efficiency standard than currently expected.
Replacement created a major headache for the building’s owners - removal of the roof structure from any building is a major project on its own. Doing this is a narrow yet busy Street in the West End of London just to renew mechanical plant would have easily trebled the base cost of replacement chillers. However, NuChill® offered a ‘through the building solution’. ThermOzone have been supporting the chillers since 2010 so were familiar with the site and indeed during this period had replaced major components several times. Thus it was obvious 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, e.g. these 250kW 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 most 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 the volume of the condensers, plus maximising the airflow capacity of the 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, minimising local disturbance and saving the massive on cost and inconvenience of using cranes in a busy area of the West End. By undertaking the least reliable chiller first service was maintained throughout the project, conducted between February and August 2017. The first year’s 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.
© Trevor Dann - February 2018