Recent Projects

ReChill® upgrade to Carrier Chillers

This project concerned two large capacity (900 kW) water chillers which provide cooling for the building’s air-conditioning services. The chillers were approximately 10 years old, but were suffering from repetitive condenser failure, and were also the most significant load upon the building’s electrical supply.

Working with the Client’s Consultants, Hilson Moran, ThermOzone were tasked to come up with a dual solution to both the reliability issue of condenser failure and also to show a significant energy saving.

The former was achieved by replacing all aluminium extruded style condensers with conventional copper tube and fin condensers. The extruded condensers were introduced in the early 2000 to save both cost of materials and reduce refrigerant charge in the chiller, but unfortunately for many chillers they have proven insufficiently resilient, suffering from acidic corrosion from external contaminants notably traffic dirt. This site first experienced failures just after the original warranty and since had replaced numerous individual condenser panels year on year (each chiller has 14 condenser panels). Any panel leaking resulted in a substantial if not full loss of system refrigerant, so although the newer style uses less refrigerant in service, repetitive leaks mean a very expensive and non environmentally friendly chiller. Using R134a the refrigerant has a CO2 tonnage equivalency in Global Warming terms of ~ 1,430 /kg, so although not providing direct energy savings form this element of the project with some 80 kg per system, the CO2 atmospheric loss represents some 114,400 tonnes of CO2. This placed a huge onus upon the chillers owners and users to act effectively.

However, during our preliminary discussions with the Consultants it became clear that these chillers were mostly lightly loaded, and as such were running with the compressors only at a partial load condition, with the specific load being controlled by an internal slide valve device, a simple mechanical slide which varies the amount of screw profile actually providing compression. Simple this mode might be, but efficient it is not. As soon as a compressor slide valve moves away from its optimum 100% full load position its operating efficiency falls away.

Like many other industrial motors significant gains in efficiency can be achieved by applying inverter variable speed / variable frequency drive inverters to screw compressors. Use of the inverter allows the specific load to be controlled by shaft speed variation rather than moving the slide valve away from its optimum position at 100% (mechanical) capacity.

ReChill Upgrade - Gracechurch St Graph

On most chillers with two systems using an inverter screw compressor can achieve a decent turn down ratio of ~ 20%, yet due to improved (lower) condensing load as the capacity required reduces, plus improved rotor lip sealing achieved as a result of the progressively slower moving shafts (the oil sealing the shafts has more time to achieve a better seal and the cross rotor pressure differentials reduce). So with the compressor at full speed in either mode a standard level of efficiency is achieved, but with slide valve control this progressively reduces as the compressor unloads, and will have lost ~20% efficiency at the 50% load point, when its part load efficiency then falls off a cliff, barely
attaining 50% of the efficiency at <35% slide valve load.

Comparing this with a speed controlled shaft from the same standard point, as the load reduces the efficiency markedly improves, indeed measured using the Building Regulations Part L formula for assessing Seasonal Energy Efficiency Rating (SEER) for water chillers the overall SEER generally improves from ~3.8 with a conventional slide valve screw compressor to ~4.8 with an inverter screw, or in energy saving % terms ~20% improvement.

However, at partial load the energy savings become outstanding, with the specific EER of the slide valve screw reducing as low as 2.5, whilst the inverter screw can achieve as high as 6.0. Comparing these efficiencies shows an energy saving at minimum load >50%.

In practice most chillers will spend over 90% of their operating time in the part load operating mode, and whereas this might not be at the minimum position typical savings on many UK sites where cooler conditions place us beneath the standard SEER curve prediction, we are seeing retrofit and new chillers utilising Inverter screw load control ~30 - 35%.

Gracechurch Inverter application

When we looked closely into the option to retrofit the chillers to Inverter we studied the OEM manuals for both the chiller and specifically the model of compressor assigned. It was discovered the OEM had designed these compressors with slide valve load control also to be used with VFD Inverters. So it was then simply a case of sizing the appropriate capacity inverter, and re-designing the controls with a strategy to suit.

Because the original controls had no feature to allow for inverter mode load control this meant we had to replace the main chiller controller as part of a package of works. However, because we were able to retain the original compressors, the full retrofit cost was quite reasonable and certainly complied with a decent payback requirement.

Since the project was concluded in early 2017 to the second chiller we have seen consistent energy savings throughout the operating seasons.

The original primary reason for the project has been completely successful with the use of conventional condensers.

Screw Compressors & Load Control by Variable Frequency Drive Inverters

Gracechurch Street Chiller - ThermOzone

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 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.

© Trevor Dann - December 2017

NuChill® Chilled Water Pump Upgrade

Following the successful NuChill® chiller replacement project undertaken in 2015-16 ThermOzone had identified that the existing chilled water circulating system was not providing very effective service, and was less than efficient.

Following discussions with the project Consultants, Mssrs Arup, we conceived a simple solution that resolved two issues, and has seen the project become self funding within a few years.

The client on Leadenhall Street operates from a medium size office building in the heart of the ‘Square Mile’ of the City of London. The chillers project here was our first full retrofit NuChill® project. It made sense to optimise the overall chiller performance by paying attention to the mode of chilled water flow control.

To properly understand the issues identified and how we resolved them is broken down into several stages…

Primary Loop System

Modern buildings using chilled water as the mechanism to provide a cooling source for the various services around the building need to optimise how the system is laid out. A variety of services require a continuous supply of cold (chilled) water, typically around 10 - 12oC. However, getting the water to diverse secondary systems at an even temperature and with sufficient flow can be difficult. The typical resolve for this is a dual loop system. The first or primary loop simply circulates water using a pump (and normally a standby pump) around a loop whereby the chillers, normally at least two, but any number can be provided, are connected in parallel and downstream of the pump.

From this loop at some point after the chillers’ outlet pipes have re-joined, further take off pipes or headers will provide water to separate ‘secondary’ pumps which in turn send the water to individual services, e.g. Air handling units, Fan Coil units and chilled beams.

These systems are generally quite diverse, and operating a Primary / Secondary loop system makes the task of cooling the water in a controlled manner far more manageable.

The flow around these secondary services picks up waste heat energy from the building then returns the warmed water back to the primary loop, and this water is in turn re-circulated through the chillers for re-cooling. This is a continuous process. It is also self regulating - if the secondaries draw less then the flow in the primary loop will simply cool at the return to the on line chillers, reducing the load by the chillers own internal control.

Mixed flow restriction

An inherent problem with parallel flow chillers serving the primary loop is that to achieve a steady flow temperature to the secondary system headers means either all chillers must be running, or if one or more shuts down when the load is light, then the running chillers must compensate by further (over) cooling the water passing.

The causes a further problem because having a variable setpoint for individual chillers is complex to control, and the further the water is cooled the more energy is consumed by the chiller.

Indeed there is also a fundamental limit to how cold the water can become before freezing becomes a significant risk. Practically for most systems this is 6oC, but why should a system that is overall required to supply water cooled to 10oC have to over cool some of the water to 6oC.

A further explanation of Mixed flow temperature dilution

If the primary flow is say 100 litres per second, entering the chillers at 14oC, with a desired mixed flow temperature of 10oC, and this is split into two equal parts flowing through each chiller. Only Chiller 1 is running, but water is circulating through both chillers.

Chiller Type Temperature
Chiller 1 & 2 return temperature 14oC 
Chiller 1 exit or flow temperature 10oC
Chiller 2 exit or flow temperature 14oC
Mixed flow temperature 12oC average of each chiller flow temperature.

Thus is can be seen that with both chillers running the Mixed flow will be 10oC as desired, but as the load falls off and one chiller stops, the mixed flow then rises to 12oC, which is less than desirable because the secondary services still require water fed at 10oC.

Thus to compensate Chiller 1 now has to cool the temperature to 6oC, reducing the average Mixed flow to [(6 + 12) / 2)] 10oC.

The more chillers that are assigned to a primary loop the more critical this problem becomes, and some chillers with multiple sub-systems can see compounded mixed flows. Indeed one site we recently observed was requiring water at 6oC with four chillers in a parallel connection. To achieve 6oC Mixed flow but with just one chiller operating would have required it to supply water back to the system at -12oC, a somewhat difficult task, and even with anti freeze -12oC is a difficult and expensive temperature to cool to.

Back End Valves

There is however a simple solution to this problem - the Back End Valve. This is a motorised valve that can shut off the flow when the chiller is not required. This means the mixed flow now equals the temperature from the individual chiller/s remaining on line. This removes the need to overcool the water, increasing the overall operating efficiency, and also requires less effort from the primary circulating pump, saving pump energy.

Pump control - PID loop

As a chiller shuts it back end valve, less water circulation is required in the primary loop. Indeed the total flow in the loop to the secondary headers is less important than a maintained constant temperature.

A proportional–integral–derivative controller (PID controller) uses a feedback loop signal to accurately control an output, in this case the speed of the chilled water primary circulating pump. The logic is provided inside the pump Variable frequency drive Inverter, and requires just one control input variable to achieve accurate a constant control based upon a continuous pressure increase across the pump.

As an individual Chiller’s Back End Valve closes, the flow around the system becomes restricted, imposing a greater back pressure to the pump outlet. The pressure sensor detects this increase and the PID control loop within the Inverter smoothly and instantly adjusts the pump speed to reduce the pressure difference to the setpoint assigned. There is no requirement for any complex control logic between the Chiller’s and the pump, the water flow and pressure inter-reaction provide al the control input required.

A separate run / stop logic is required to ensure the pump cannot run ‘dead headed’ i.e. in a situation where no chillers are required, and this is a simple task for the BMS system assigned to stop and start the overall systems, using end switch contacts on each Back end valve. At least one back end valve end stop “Open” must provide a positive signal to allow the pump to run.

A separate control loop is also provided within the BMS to rotate Duty and Standby pumps and to run the standby pump in the event of failure of the duty pump.

Project Concept

Leadenhall Street Pumps - ThermaGroup

So in consideration of the above the actual pump set up at the site was from some elderly belt driven fixed speed pumps. The permanent 3 phase current draw was ~19 amps (~11.5 kW). Closing the Back end valves would see the pump having to work harder to overcome the increased back pressure, so use of Back end valves was not practical without including output control.

Modern Inverter pump selection tables showed we could achieve similar flow requirements for far less effort, and the pump re-selection suggested a run current of just 6 amps (~3.5kW) would support the same flow requirement of all three chillers at this site. It is recognised a significant energy loss is caused by the belt drive, however this was the expectation at the maximum flow position with all three chillers back end valves open. As the load reduces to two (95% of the time) or one chiller (30% of the time), the pump energy would also reduce, although not by so much.

Armstrong offered a Pump with very similar geometry. This minimised the modifications required to the pump connections, and this was installed complete with a suitable VFD Inverter with built in PID control capability, plus a simple pressure sensor allowed us to achieve accurate control from this single input. Please note the single sensor works on a principle that the low side pump inlet pressure is held constant by the pressurisation system.

The whole thing can be integrated separately to the BMS system as an added feature, although for initial set to work and proving the BMS works were of secondary importance.

Project implementation

As this is a live and operational site we had to achieve integration with minimal loss of service, indeed because each pump set had isolation valves this was quite straightforward.

Some minor pipe adjustments were necessary but all achievable within a single day for the hot works (welding) works.

The electrical supply and control for the original pumps was largely re-used. The contactors were effectively removed and replaced with the Inverter as Pump power controller and starter. The original pump run and fault signals were used via a control relay to provide the start signal, with the VFD Inverter providing the fault output function.

© Trevor Dann - December 2017

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.

Our client on Old Burlington Street operates from 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.

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, 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

Old Burlington Street - ThermaGroup

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 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.

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 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

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

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 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.

Associated works

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

Compressor Repair for Transport Ships

Compressor and Controls for Transport Ships - ThermaCom

Marine ships are floating “machine” and are in need of durability with a high level of reliability often against technology, regulation and environment when actively at sea.

Servicing Air-Conditioning on Modern Trains

Air Conditioning Servicing and Maintenance - ThermaCom

Modern train carriages are effectively sealed capsules. Maintaining a comfortable environment is essential and is the principal reason for passenger complaints after late trains.

Chiller Repair & Refurbishment

Repairing and Refurbishment Rather Than New Equipment - ThermOzone

Woolverstone House, Berners Street, London.

Secondary ReChill® upgrade to Carrier Chillers

R134a & Inverter Upgrade project

lombard street 1

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.

Leadenhall Street - ThermaGroup

Our project in Leadenhall Street was the first full NuChill® retrofit. 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

ReChill® upgrade to York Chillers - An eminent Financial Services Organisation in Mayfair

This case study concerns the progressive upgrade of two York chillers installed at the clients premises in Mayfair. For commercial reasons we have been requested not to name the organisation within this Case Study, however we understand this will be endorsed by the Client contact.

The chillers as now appointed are using state of the art Variable Frequency Drive Inverters in conjunction with the Screw compressors and controls installed over the past few years.

The energy expectations reflect those achieved at similar prior projects, demonstrating the repeatability of ReChill®, and option for selective application of individual concept techniques to suit Clients budgetary constraints.

This site is equipped with two York YCAS range 468 kW air cooled water chillers, installed we understand ~ 2000. Our involvement commenced in 2010 with initial works to replace compressors provided by the original manufacturers following service failure, prior repair attempts and other chiller issues. The new Screw compressors we have provided immediately proved to be more effective, efficient and reliable than those replaced, however other issues were highlighted concerning the Control and in particular the poor condition of the air cooled Condenser assemblies, so further works were then authorised for us to apply other ReChill® concept techniques on an ad-hoc basis.

Eventually in 2013 works were authorised to correlate the prior upgrades into a concise upgrade to a final ReChill® standard whereby by then all Compressors; Controls; Condensers and new EC Fans had been assigned. The chiller was from new using R407C, based upon the base design originally for HCFC R22.

At that time (2013) Inverter upgrade was in its infancy, with the primary commercial driver for projects being a necessity to maintain service with minimal site inconvenience.

Hence for the earlier phases of the project energy assessments were not a key part, although based upon similar projects to similar chillers e.g. HBOS, 33 Old Broad St, we would surmise these progressive upgrades likely achieved ~ 35% energy savings, in addition to providing effective reliable service.

2015 - R134a & VFD Inverter Upgrade

This year the project just concluded is application of the techniques already pioneered at several sites with use of Inverter Screw compressors, but with the considerable plus of taking an original R22 design chiller, assigned since new upon R407C, and then converting this to R134a.

So why R134a? & Why wasn’t R134a used in the first place?

R134a is a simpler refrigerant and indeed is a base refrigerant for many of the R400 series ‘blends’, including R407C. For those who understand the thermodynamics of refrigeration R134a has no ‘glide’, and this correlates with the fact it is per unit of input energy ~ 8% more effective at transferring heat energy than R407C.

But R134a has a considerable disadvantage over R407C inasmuch as its specific enthalpy (energeticness) is markedly lower than R407C, e.g. per unit volume of gas compressed it transfers ~ 30% less heat energy. This means any particular chiller using R134a will require a larger compressor, and to a lesser extent a larger evaporator (evaporator load loss ~ 10% - see below).

Installing a compressor to replace the original with one suitable for an extra 30% pumping capacity means a more expensive and larger compressor, and for many chiller frames is simply not a viable solution.

However, along comes the VFD Inverter. Not only does the inverter allow us to accurately speed control the compressor drive motor for improved load control and efficiency, but we can also overspeed the compressor, within sensible limits, allowing us to achieve a higher pumping rate, but without installing a larger compressor.

Practically the compressors we use can operate safely up to 70 hz, from their standard operating speed of 50 hz, and with a turn down to 30 hz. This allows us to achieve ~ 95% of the original capacity of the R407C compressor (at 50 hz) with the compressor now at 70hz R134a. The gain is all positive and at maximum speed we gain a reduction of ~8% input power over the R407C application.

But it does not end there…

In the pre-conversion compressor load control was by slide valve modulation, with a progressive loss of efficiency (effort achieved : effort in) of approximately 15% from its most efficient at maximum slide valve load to a minimum of ~ 40% slide valve load.

Now with Inverter providing operating speed as principle mode of load control, the part load trend actually increases efficiency as the compressor unloads (runs slower), with maximum efficiency now achieved at the minimum load speed.

This effect is further boosted again by the ambient operating conditions of the chiller as a whole, because as the ambient temperature falls the efficiency increases, and inverter screw compressors make best use of this phenomenon.

Notably compressors rarely operate at either end of the load scale full load or minimum load, but tend to operate ~ 1/3 between these two extremes. Meaning for most users they will experience a gain of ~ 10 - 15%, plus the gain from the refrigerant change to the more efficient R134a ~ 8%, so an overall gain ~ 18 - 23%.

R134a has some other notable advantages for the clients…

  • Lower Global Warming CO2 Tonnes per kg R407C = 1,774; R134a = 1,430
  • F Gas HFC phase out replacements for R134a developed already
  • Lower operating pressures places less stress upon retained components

Evaporator load loss

A typical evaporator will lose approximately 10% cooling capacity when taken from R22 / R407C to R134a. However, for most systems the rating point for the chiller is the capacity of the compressors, with ~5% spare capacity within the evaporator. Generally it is reasonable to assume this as ~ 5%, although in the case of this York chiller we know from the maker’s data that the same evaporator assembly is used in the 468 kW (the units at this site) and the 507 kW, so our assessment of evaporator capacity loss is 507 kW - 10% = ~ 456 kW. Effectively the client loses ~ 12 kW capacity at the very top end. This capacity loss is explained in our proposals and for this site is negligible compared to the significant gains in energy terms.

Life expectancy

The principle upgrade works to correlate to a proper ReChill® standard was concluded for both site chillers in 2013, for which with the retained evaporators our projection for life expectancy was 10 years +. Two years on the recent upgrade has not reset this figure but we are of a view that the actual lifespan could easily exceed this nominal period as long as regular PPM is applied. The works have been provided with a one year warranty, extended to include the compressor motors, alleviating any risk to the client resultant to the inverter drives themselves.

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 benefits
  • Duty for Duty the turn down ratio is ~ 40% system load, or 20% chiller load
  • Reduced rotor tip blow by at lower speeds 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.

Associated works

ReChill Upgrade - ThermaGroup

The latest phase of upgrade utilised the Compressors & Controls systems previously installed up to 2013, so price for this project was that to supply the New Inverters and configure the chiller hardware and soft systems to accommodate this technology.

Target Reset - The final cherry on the cake for energy optimisation

‘Target Reset’ allows us to adjust the demand setpoint to match actual conditions. Many buildings simply set the desired temperature and let the chillers get on with it. However, cooling the water to 6oC uses energy and costs the clients money, so if in cooler weather 10oC or even 12oC will satisfy the load then why not adjust the setpoint. ‘Target Reset’ does this automatically using the chiller’s own ambient temperature sensor as a reference, with opposing indices as Ambient rises through a selected scale, the chilled water demand setpoint reduces through another separate scale - fully automatic and presents up to 15% energy savings or perhaps higher when the clients do not need their chillers to work so hard, and in some cases not at all (100% energy saving on those days !).

Rodem® Remote Access and Virtual Servicing

The site adopted Rodem® earlier this year to allow external access to the chillers for servicing, monitoring and breakdown purposes. Rodem® allows us to scrutinise individual chillers on a monthly basis, and importantly means we can ‘look’ at the chiller at its most crucial load points - hottest day of the Summer, coldest day of the Winter. We can check the operating efficiency and compare this to an establishing datum point. If there is a sudden change in efficiency then the client might be ignorant, but his electricity meter will suffer. Rodem® allows us to instigate early corrective actions, often before the client himself becomes aware of an issue.

If breakdown does occur for any reason then we can log in to the chiller within minutes and on most occasions can diagnose and resolve issues without need for the consequent delays, expense and inconvenience of waiting for a Service Engineer.

© Trevor Dann - December 2015