CPD PROGRAMME
Free cooling hybrid chillers There are a number of free cooling and adiabatic chillers on the market today (including the one illustrated in Figure 3). Some have screw compressor technology and inverter drives with electronically controlled heat rejection fans, to try to reduce the high start up currents and high part load running currents, while others incorporate the centrifugal, oil-free, magnetic bearing compressor package already mentioned. These have a combination of both free cooling for low ambient operation and adiabatic cooling for high ambient scenarios (hence, ‘hybrid’). The compressors have particularly low starting currents of about 5 amps per 350 kW cooling capacity. Seasonal EERs greater than 12 are easily achievable with these chillers, plus the higher chilled water temperatures allowed with the environmental envelope recommended for data centres. Table 1 gives an indication of the performance for the different options in which the chiller can operate. A further development to this hybrid
chiller is the addition of an actively managed evaporative system. Water is absorbed by a porous natural-fibre honeycomb array in the direct air path of the coils. The adiabatic cooling effectively reduces ambient temperatures in the immediate vicinity of coils by up to 10C, lowering condensing temperatures and significantly improving the chiller’s energy performance. The adiabatic advantage also increases chiller capacity at peak load conditions, enabling it to cope with high ambients that might otherwise overwhelm a standard chiller. The system can be set to activate automatically at a pre-determined external temperature. Water consumption is low. In UK conditions, £600 worth of water consumed a year results in energy savings worth some £8,000. In summary, the energy savings and improved performance also provide the following benefits: l Extended working life due to reduced compressor run-time;
l Reduced service and maintenance, due to reduced run-time;
l Payback time further reduced; and l Extended chiller capacity at peak load, enabling it to cope with extreme ambients that would defeat other chillers.
Floating head pressure The system operates with a floating head pressure, providing opportunities for savings not available to conventional
www.cibsejournal.com MW UV filter Cooling tower or adiabatic cooling unit CH.W Water cooled Turbocor high efficiency chiller Figure 4: Water cooled Turbocor chiller with adiabatic cooler
designs. Unlike standard chillers that have fixed head pressure, this centrifugal compressor package constantly self- regulates and optimises its performance in response to ambient conditions and load.
Water cooled options Where internal plant room space permits and/or noise design criteria is exceptional,
Duty (kW) Free cooling (kW) 900 900 900 900 900 900 900 900 900 900
0 0
229.6 460.6 870.8 900 900 900 900
894.6
Duty (kW) Free cooling (kW) 900 900 900 900 900 900 900 900 900
0 0 0
344.4 694.4 900 900 900 900
Duty (kW) Free cooling (kW) 900 900 900 900 900 900 900
0 0 0 0 0 0 0
water cooled versions of these low energy chillers can be used, matched with closed circuit cooling towers or adiabatic coolers. Figure 4 illustrates a typical system. Although doubling up on plant items, water cooled versions of the oil-free technology can achieve ESEERs (European Seasonal Energy Efficiency Ratio) in excess of 10, while using the higher allowable chilled water temperatures and capitalising on adiabatic cooling at higher ambient conditions. Centrifugal, oil-free chiller packages up to 2.4 megawatts (MW) are available. Lower condensing water temperatures can be achieved with this equipment and will produce a very efficient overall cycle. Lower starting and running currents for this type of equipment can also reduce infrastructure cabling cost and maximum demand charges, as well as running cost.
© Terry Welch and Steve Chaplin (Klima-Therm) 2012
References
1. ASHRAE TC 9.9 white paper 2011 Thermal Guidelines for Data Processing Environments
2. Fresh Approach, CIBSE Journal April 2011, Marcus O'Brien
3. Klima-Therm 2011 Adiabatic Free Cooling Options for Data Centre
TMA 3A900B EC FC – Adiabatic system and free cooling EER (–) 4.8
5.45 8.73
12.33 22.96 22.96 28.32 75.63
142.86 257.14
39.2 39.2
31.78 11.9 6.3 3.5
35 30 26 22 15 12 10 5 0
-5
TMA 3A900B EC FC – Adiabatic/dry mode only and free cooling EER (–) 4.43 4.8
5.45
10.39 16.95 28.32 75.63
142.86 257.14
187.4 165.2 86.6 53.1
31.78 11.9 6.3 3.5
35 31 26 20 15 10 5 0
-5
TMA 3A900B EC FC – Spray/dry mode without free cooling EER (–) 4.43 4.8
5.45 7.06 9.01 9.94
10.23
187.4 165.2
127.48 99.94 90.56 88
Et. glycol 20%; RWT= 26C; LWT= 20C; Reference capacity 900 kW Table 1: Typical energy savings and EERs for hybrid chiller
April 2012 CIBSE Journal 53
35 31 26 18 10 0
-5
Input (kW) Ambient temp. (C) Working condition 187.4 165.2 103.1 73
Evaporative cooling Evaporative cooling Evaporative cooling Evaporative cooling Evaporative cooling Dry mode Dry mode Dry mode Dry mode Dry mode
Input (kW) Ambient temp. (C) Working condition 203
Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode
Input (kW) Ambient temp. (C) Working condition 203
Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode Dry mode
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