88 Leadenhall Street 2016

88 Leadenhall Street

NuChill® upgrade to Carrier 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.

88 Leadenhall Street was our first full retrofit NuChill® project. The site is a high specification office building in the heart of the City of London. Cooling was provided by three tiring Carrier 30GX range chillers dating from ~2004. Although relatively modern, the technology applied using geared screws was not very efficient and the site was plagued by nuisance trips and equipment failures, which were both inconvenient and expensive.

The re-design ThermOzone conceived replaced all functioning assemblies using selected high quality components and sub-assemblies throughout. We also assigned the well established Inverter Screw compressor as primary drive mode along with other energy efficient features - rifle tubes condensers, EC Fans, Electronic expansion valves and associated modern controls. A separate follow up project has also been included to replace the elderly pumps with modern inverter pumps, but this will be covered under a separate case study.

Crucially for the Clients benefit the entire renewal process was undertaken via the pedestrian access route, saving the massive on cost and inconvenience of using cranes is a busy area of the City. By undertaking the least reliable chiller first service was maintained throughout the project, conducted between January 2015 and February 2016. The first full year’s operation has allowed for seasonal adjustment during the soft landing phase.

Screw Compressors & Load Control by Variable Frequency Drive Inverters

ReChill® 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 ReChill® 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 benefitsDuty 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.

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.

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 - September 2016