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Let it snow

Upgrading the cooling equipment at an indoor snow park required a creative solution, with a number of factors to consider, says ThermaOzone operations director Trevor Dann

The SnowDome in Tamworth is an indoor real snow and ice rink complex, featuring a 170 m-long ski slope and three Academy snow slopes, as well as an ice-skating rink.

The cooling equipment providing the necessary refrigeration circulates a brine solution at -16 deg C, with the chillers providing the brine cooling packs built in the 1990s and driven by York piston compressors, using refrigerant R22 single stage compression. Condenser cooling uses water with evaporative coolers.

Design factors for each chiller:
n Brine temperature - 16 to - 18 deg C

  • Evaporating temperature - 24o deg C
  • Condensing temperature + 26o deg C
  • Cooling capacity 370 kW
  • Key problems identified – plant unserviceable with refrigerant R22; ageing condition of compressors and controls; poor efficiency of
    EER 1.6.

In respect to options for the upgrade, new plant was the comparative choice.

However, this comes with immense cost and disruption.

So instead it was decided that the goal was a further decade’s service from the bulk of the existing plant, notably the large evaporator and condenser vessels and base support structure.

Regarding refrigerant choice, it was clear R22 had to go. The choice also needed to take into consideration the future availability of the chosen refrigerant and, in particular, its GWP factor.

The F-Gas directive review makes GWP factor a significant consideration.

With that in mind, the best alternative, R134a, would result in major loss of capacity, while R407C, although effective for at higher temperatures, was not proven nor recommended at the operating temperatures required.

Therefore, R407F was the next best available and well is established as a medium- to low-temperature refrigerant as a replacement for R404A, which has one of the highest GWP factors.

Our wider experience shows that for fluid chillers, screw compressors consistently deliver a more effective performance in terms of both efficiency and reliability than piston compressors, due to the improved volumetric efficiency and fewer moving parts.

TinyTag Energy and Temperature Loggers were installed on the chiller for several weeks, allowing the actual energy consumption to be measured.

This was then assessed against system parameters to determine the operating efficiency to be 1.6 EER.

A further enhancement to allow use of smaller compressors and maintain efficiency to lower load was by application of inverters.

Assessment of predicted capacity meant the selected compressors could deliver consistent energy benefits across the load range.

Heat exchangers

Because the inverter can run the compressor faster than the supply frequency limit of 50hz, the compressor can deliver service to 63 hz. The selected compressors were assigned with economiser 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, therefore improving the heat exchange within the evaporator.

2. The refrigerant used to perform this cooling function 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. An efficiency gain of 10 per cent is achieved.

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.

ThermaCom also 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.

The Magnum controller provides all the control functions required, including direct control of the Electronic Expansion Valve.

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 system is equipped with Rodem, providing remote access via the internet. Operating levels and set points can be viewed and adjusted and optimised. Ongoing use of Rodem will allow us to keep close tabs upon the operating systems, and in particular to ensure the efficiency is maintained.

Energy efficiency Improvements 

  • Screw compressors in place of piston compressors
  • Variable frequency Inverter drives
  • Economisers
  • Electronic Expansion Valve
  • Complex Controller
  • Operating Energy Efficiency Ratio EER peak load 2.5
  • Nominal conditions               370 KW @ 26oC 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 2.7 results in 41 per cent energy savings.

Capacity change and efficiency benefits

  • Original system:    Rated at: 370 kW >  370 - 400 kW     EER   1.6 > 2.5 - 2.7
  • The EER comparison equates to an energy saving of 41 per cent
  • Input power before 231 kW / after 137 kW – saving 94 kW
  • This plant runs 24 / 7 for most of the year, with a typical load above 90 per cent. At 90 per cent 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 22 deg C) reducing load on the other chiller units.

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