NuChill® upgrade to Trane Chillers

NuChill^® Upgrade to Trane 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 serviceable, perhaps needing a facelift, but they provide a superb platform to install the new (re) designed chiller. NuChill^® also offers full bespoke chillers that fit into difficult locations where an off the shelf ‘box’ will simply not fit.

Grosvenor Street – Mayfair

Grosvenor Street, in the heart of Mayfair, just off Bond Street, is a prestigious address and office location for leading firms operating in the financial sectors. This project was for the water chillers providing cooling to the building’s air-conditioning systems.

For the past ~17 years the air-conditioning cooling source for the building has been provided by two middle age but increasingly unreliable and inefficient Trane water chillers situated in a roof top plant area. These were installed during the building’s major refit in 2003. Now past their original 12 year life point the chillers had become less reliable, and were costly to run, being of a much lower efficiency standard than currently expected.

Replacements created a major headache for the site – heavy chillers of ~270 kW cooling capacity with access only from a very busy traffic junction. Cranage just to renew the mechanical plant could have easily doubled the base cost of the replacement chillers. However, NuChill^® offered a ‘through the building solution’. ThermOzone have been supporting these Daikin / Trane generic 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 an opportunity to improve both energy efficiency and resiliency.

Efficiency

Inverter driven and controlled screw compressors; electronic expansion valves; EC fans; high efficiency condensers; compact and high efficiency evaporator.

Resiliency

The original capacity was maintained but the re-design allowed us to reset this to a higher performance point. These 270kW chillers were originally sized for ~32^oC 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 35^oC. Indeed it is forecast the chillers will remain serviceable to an air con temperature in excess of 40^oC, albeit with some loss of capacity above 35^oC. 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 on top of the condensers.

As part of the NuChill^® design ThermOzone conceived and replaced all functioning assemblies using selected high quality components and sub-assemblies throughout. The retained chassis not only removed the need to crane in replacements, but provided a significant element of recycling by re-use. We were also able to achieve the desired capacity within the same space frame.

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 a neighbouring site to move some of the components, minimising local disturbance. By undertaking the least reliable chiller first, service was maintained throughout the project, conducted between December 2019 and April 2021. The first year’s full operation is allowing for seasonal adjustment during the soft landing phase.

COVID-19 Project Interruption

The initial COVID-19 lockdown saw this one amongst many other premises decide to shut for normal business. This happened when the first chiller was partly processed. However, when works resumed we maintained service through what was a very warm Summer during 2020, with chiller 1 running by early July, post lockdown where it handled the entire summer load without any fuss, not even a spurious trip.

At the sites request, the second chiller was postponed into the Winter 2020 / 21 season, but in fact the resilience of chiller 2 was not needed, indeed by the time we tackled chiller 2 any back up had anyway disappeared.

Screw Compressors & Load Control by Variable Frequency Drive Inverters

ReChill^® has proven that 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, 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 equipment fully designed from scratch. 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 ~5 – 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 Inverter screw compressors replacing fixed speed scroll compressors provides an energy saving linked to the lesser pumping efficiency due to motor losses of the scroll compressor compared with the screw equivalent displacement ~35%.

With VFD Inverter control the load control Slide Valve is only used for soft start purposes and is then held at full load position, optimising the screw compressor at its most efficient. By simply controlling its speed 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 ~35% 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.

Associated Works

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 EC Fans 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.

Submission for next Carbon Trust Re-Accreditation February 2022

Some 9 months from 1st sub-project completion the following is a medium term appraisal of the NuChill^® project.

Since the upgrade the first chiller has performed exceptionally well with no issues at all, with nil teething period trips.

Regular servicing of the chillers has continued under ThermOzone jurisdiction and has allowed the clients to benefit from an extended warranty of three years. During this initial first year period there have been nil major nor indeed minor equipment failures.

Servicing cost is similar to the servicing costs prior to the project, with the addition of RODEM^® remote virtual servicing, allowing the chillers to be inspected for issues and control settings adjusted, and for resets to be applied remotely as required.

RODEM^® also allows seasonal optimisation, maximising operating efficiency at all climatic loads.

Energy study

The prediction is based upon an original EER of 2.5 (determined from local data logging measurements). This provided for a more favourable starting point for the original specification of the chiller where available maker’s data provided an EER ~ 2.2. The enclosed pre-project energy savings data logging & prediction was undertaken in May 2019.

As part of the proposals we issued a prediction of predicted (post-project) SEER, for which we modelled to the building regulations part L SEER rules of assessment, although adjusted to the ESEER (Eurovent) standard. This predicted energy savings varying with specific ambient conditions ~50% to 56% (the higher % saving being achieved at lower part load). We expected to achieve ESEER of ~5.0, equating to ~50% average energy savings.

As an aside it was expected that we would repeat the energy data logging after the project to compare actual with predicted energy usage. However, since COVID-19 the building usage has considerably reduced, and with it the local chilled water load demand. Thus it could be misleading to repeat the data logging without similar external load conditions. The situation for a further data logging session will remain open pending building use returning to a more normal situation.

Summary Case study compiled by:

© Trevor Dann – May 2021

Posted in the Recent Projects blog category on June 24, 2021.