The SnowDome May 2015 Tamworth Low Temperature Fluid Chiller ReChill® Upgrade Project Synopsis.
The SnowDome in Tamworth is an indoor real snow ski slope and ice rink complex.
The cooling equipment providing the necessary refrigeration is an indirect system using a brine solution circulating at approximately -16oC. Three chillers provide the cooling of the brine. Of these two are Star Packs build bespoke to application in the 1990’s and driven by three York JS series piston compressors on a single circuit, using HCFC refrigerant R22 single stage compression. Condenser cooling uses water with external evaporative coolers. The third chiller is a newer Multiple Screw compressor pack. Design factors for each Star Brine chiller: Brine temperature - 16 to - 18°C, Evaporating temperature - 24°C, Condensing temperature + 26°C, Cooling capacity ~ 370 kW.
Key problems identified
1: Refrigerant R22 meant the plant was effectively unserviceable after 1 January 2015.
2: Plant age particularly compressors and ancillary modulating components.
3: Operating efficiency - operating E E R assessed as ~ 1.6 Options for ReChill® upgrade. New plant was the comparative choice, but presented both immense cost and disruption.
The goal was to provide a further 10 years service from the bulk of the existing plant, notably the large Evaporator and Condenser vessels and base support structure. Refrigerant choice was limited by the available ‘Direct Replacements’ for now defunct HCFC R22 operating characteristic, or possibly an HFC, but in our experience perhaps less suitable for piston compressors. Use of a ‘Direct replacement’ would only satisfy the first key problem, indeed use of such gases can exacerbate problems with worn plant. So also taken into account was the perceived future availability of the selected refrigerant which is largely affected by the GWP factor (the number after each refrigerant is the GWP CO2 tonnes calculation factor where 1 kg refrigerant = factor / 1,000 CO2 tonnes).
Include R422D 2,729 R422A 3,143 HFCs R404A 3,922 R407A 2,107 R407F 1,825 R407C 1,774 R134a 1,430. The more recent F Gas Directive makes many of these less desirable where high GWP factors arise, e.g. R404A. The best alternative R134a would result in major loss of capacity. R407C although effective for R22 at higher temperatures was not proven nor recommended at the operating temperatures required. Therefore R407F was the next best available and is established and reasonably widely used as a medium to low temperature refrigerant, indeed widely as a replacement for R404A, which has one of the highest GWP factors. Therefore in balance with the other key problems this project addresses, R407F was selected as providing a good medium/low temperature application solution with operating characteristic similar to R404A, but with far better GWP factor.
Plant Age & Operating Efficiency
Our wider experience with ReChill® has conclusively shown that for fluid chiller applications Screw compressors consistently deliver a more effective performance in terms of both efficiency and reliability than piston compressors, and this is due to the improved volumetric efficiency of the Screw compressor. Assessment of the energy efficiency of the plant as found compared against predicted energy figures for equivalent refrigeration capacity for new Screw compressors showed a considerable improvement in operating efficiency.
TinyTag Energy and Temperature Loggers were installed on the chiller for several weeks allowing an accurate record of the actual energy consumption to be measured. This was then assessed against measured system parameters to determine the operating efficiency. This was determined to be ~ 1.6 EER. Indeed the energy savings predicted showed an overall project payback within ~ 1 year, regardless of the other benefits of improved reliability and addressing of the R22 problem.
A further enhancement to allow use of smaller compressors and maintained efficiency to lower load was by application of Variable Frequency Inverter Drives. Careful assessment of the capacity predictions showed the selected compressors could deliver consistent energy benefits across the load range with highest efficiency achieved at lower base load, where traditional slide valve load control notoriously becomes much less efficient. Also an inverter can run the compressor faster than the supply frequency limit of 50hz / ~2,900 rpm, and for this application the compressor can deliver service to 63 hz / ~ 3,650 (input shaft speed).
The increase is service delivery at the higher speed is virtually directly proportional to the speed increase. However, as screw compressors slow to wards their lower operating speed limit (typically 30 hz) the lip sealing improves due to the oil having more time to form a rotor seal and lower inter-volute pressure drops. This leads to less rotor tip blow-by and higher lower load efficiency. Use of Economisers - SWEP The selected compressors were assigned with use of Economisers.
These are small plate heat exchangers that perform two energy improving functions:
1: The sub-cooling of the liquid refrigerant is increased, reducing flash gas losses downstream of the Expansion Valve. The colder the liquid refrigerant is delivered to the evaporator, the more available heat exchange can occur within the evaporator.
2: The refrigerant used to perform this ancillary cooling function is a secondary take off from the main liquid line after the condenser, and is controlled by a small conventional TEVv and Solenoid valve.
After this refrigerant has performed its task of cooling the bulk of the main liquid flow, this gas is then injected into the compressor rotors after the suction cycle has finished. This extra burst of refrigerant substantially increases the volumetric efficiency of the compressor with very little additional motor load/power input. Overall an efficiency gain ~10% is achieved.
Compressor Choice - Bitzer
Following close scrutiny of the available options with manufacture support for use with R407F and variable speed/frequency Inverter drives, we opted for Bitzer CSH 85 series compressors. Variable Frequency Drive Inverters - Rhymebus ThermaCom imported suitable 125 kW drives from Rhymebus of Taiwan. These are substantial and well appointed units offering a high level of reliability with a host of inbuilt safety features further protecting the compressor motors, and react smoothly and effectively to the external load/frequency commands.
Magnum with Sporlan EEVv. The final design aspect applied to optimise system performance was a Sporlan Electronic Expansion Valve, providing for accurate control of the superheat across the entire load range, allowing this to be set to an optimum level, and taking due account of the refrigerant blend's Glide effect. On R407F this was notable as several degrees at the critical operating area (saturated suction) ~ -20 to - 24°C.
During the tuning process a key factor has been adjusting the superheat as low as is feasible without allowing any liquid refrigerant return at the compressor. This reduces the discharge superheat, and directly increases operating efficiency. The whole set up is controlled by a new power and control system using the established Magnum Controller, this provides all main and secondary control features along with a comprehensive array of protective and monitoring functions. The system is also equipped with state of the art Remote Access under our unique RODEM® concept. This allows remote access to monitor, reset and adjust key parameters.
A ’Virtual Service’ inspection is also possible, enabling all key parameters to be checked on a weekly or monthly basis. Summary of Energy Efficiency Improvements ABOVE PREDICTIONS Screw compressors in place of piston compressors Variable frequency Inverter drives Economisers Electronic Expansion Valve Complex Controller Assessed operating Energy Efficiency Ratio E E R Peak load 2.5 Nominal conditions 370 KW @ 26°C Saturated discharge 2.7 Notably low temperature plant never operates to the routine EER expected of more common high temperature water chillers. However, improving EER from 1.6 to 2.5 results in 36% energy savings. Other aspects taken into account Oil Control Although the compressors selected have built in oil separation, at low evaporating temperatures a substantial quantity of oil carry over occurs. Therefore the original oil separation system was retained with float switch control of solenoid valves to automatically provide refill to the compressor when required. A safety feature is also included to cause a shut down if the oil level is not restored within a defined period.
As the systems operate they control individual compressor shut down on the pump down principle. Load limiting If the condensers experience higher operating conditions the compressors will automatically load limit, preventing nuisance trips on high pressure of high discharge temperatures. This is set up as a duplex system. It was found that the compressors could deliver significantly more capacity at cooler ambient/condenser water circulating temperatures, meaning that more of the overall load could be handled by the upgraded Chiller, reducing the load on the original units, and increasing further the overall operating efficiency of the overall cooling plant. However, as the ambient temperature and hence condenser water circulating temperatures increase it was necessary to reduce the maxim operating speed of the compressors. This process is fully automated. A secondary system can also operate in extreme conditions for instance if the condenser water system suffers a problem with circulation or fouling the system will load limit if the discharge pressures become too high.
Protections Numerous safety features are provided for to protect the plant from failure. HP unload, Control HP trip, Manual reset HP trip (mechanical switch), HP Relief valves; LP Unload & Trip; Low Oil level trip; Motor overheat; Over-current; No Current; Low superheat; Low compressor differential - are the key safety features. Capacity Change and summary of Efficiency benefits Original system: Rated at: 370 kW measured EER 1.6 Upgraded system: nominal ~ 370 kW measured EER 2.7 but in lower ambient conditions can provide up to ~ 400 kW at the predicted EER 2.5
The EER comparison equates to an energy saving of ~ 41 % Equating to input power nominal full load before ~ 231 kW after ~ 137 kW difference ~ 94 kW less This plant runs 24 / 7 for most of the year, with a typical load above 90% At 90% system load year round plant energy saving ~ 0.74 MWH / p.a (94kW (less) x 24 hours x 365 days x 0.9 = 741,096 kWH savings p.a.) At typical unit cost of £0.11 / unit kWH ~ annual saving £81,500 Project cost for the client was ~ £75,000 This Pay Back analysis ignores the fringe benefits of increased capacity in lower ambients (below ~24°C) reducing load on the other chiller units.
Using the RODEM® remote system has allowed us to assess the chiller operation each day since initial commissioning, and is hugely beneficial for otherwise time consuming trips to optimise the chiller’s operation. Ongoing use of RODEM® will allow us to keep close tabs upon the operating systems, and in particular to ensure the efficiency is maintained.