LOW-APPROACH CRAC/CRAH
In addition to leading the data center cooling industry in fan/motor efficiency and air-flow capacity/ton cooling, CMS also has the capability to build custom CRACs and CRAH units with very low approach temperatures.
The approach temperature being the difference between the water/refrigerant entering into the heat-exchanger (coil) and the temperature of the air leaving the heat-exchanger. The lower you can make the approach temperature, the warmer the entering water / refrigerant can be while still maintaining leaving air temperatures that meet your design standards.
For example, it is common to run chillers that make 45° water, pumped to CRAHs, with standard coils, in order to make 65°-70° air. That's a 20° - 25° approach temperature.
One way to lower the approach temperature and still maintain the required leaving air temperature, is to use a more efficient, custom coil. A 10° approach temperature, for example, would allow you to increase your water temperature to 55° and still make 65° air.
CMS has designed CRAHs with custom coils and approach temperatures as low as 6°. This would, for example, allow an operator to use 64° water to make 70° air.
Looking at the big picture, the warmer the entering water you can use, the more hours out the year you can take advantage of low-energy cooling technologies such as cooling towers, air-cooled chillers, and free-cooling coils. During the hours when low-energy cooling is not sufficient, you can significantly reduce your chiller power consumption.
As an example, consider a 1,200 ton chilled water CRAH deployment using all CMS equipment:
APPROACH COMPARISON
| Tons | EW | EA | LA | Approach | Chiller/ Pump kW | Fan kW | Net % |
|---|---|---|---|---|---|---|---|
| 1,200 | 60° | 94° | 74° | 14° | 1,325 | 291 | Baseline |
| 1,200 | 60° | 94° | 74° | 6° | 1,175 | 342 | -6% |
| Tons | 1,200 |
| EW | 60° |
| EA | 94° |
| LA | 74° |
| Approach | 14° |
| Chiller/ Pump kW | 1,325 |
| Fan kW | 291 |
| Net % | Baseline |
| Tons | 1,200 |
| EW | 60° |
| EA | 94° |
| LA | 74° |
| Approach | 14° |
| Chiller/ Pump kW | 1,175 |
| Fan kW | 342 |
| Net % | -6% |
The kW and percent comparison is net, including increased CRAH fan kW due to higher pressure drop of the lower-approach coil. This is the efficiency benefit during hours that the chiller is running.
In certain climates, there are further benefits to realize with higher water temperatures by increasing the number of hours that can be operated using a water side economizer. Also consider that, for new construction, the installation of air-cooled chillers also eliminates first-cost and field-work related to cooling tower installation and piping. Ongoing water and maintenance costs are also alleviated.
