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.
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.
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