The selection of temperature ranges can affect the chiller plant operation and energy usage. The limiting temperatures are the required supply air temperature and either the ambient wet-bulb (water or evaporatively cooled chillers) or dry-bulb (air-cooled chillers) temperatures. Once these have been identified, the HVAC system must operate within them.
1.0 Supply Air Temperature
The chilled water supply temperature is tied to the supply air temperature. The chilled water temperature must be cold enough to provide a reasonable log mean temperature difference (LMTD) (Refer to Daikin Applied’s AG 31-002, Centrifugal Chiller Fundamentals, for more information on LMTD) for a cooling coil to be selected. Traditionally this has resulted in a 10°F approach which, when subtracted from 55°F supply air temperature, has led to the 44 or 45°F chilled water temperature. Lowering the chilled water temperature will increase the approach allowing a smaller (in rows and fins and hence air pressure drop) coil to be used. It will also increase the lift that the chiller must overcome which reduces the chiller performance.
The air pressure drop savings for small changes (2 to 4°F) in the approach do not generally save enough in fan work to offset the chiller penalty. This is particularly true for VAV systems where the pressure drops inside an air handling unit follow the fan affinity laws. The power required to overcome the coil pressure drop decrease by the cube root as the air volume decreases. A 20% decrease in airflow results in a 36% decrease in internal air pressure drop and a 49% drop in bhp.
It is sometimes suggested that the chilled water supply temperature be 2°F colder than the supply water temperature used to select the cooling coils to make sure the “correct” water temperature is delivered to the coils. This is not recommended. For a 10°F chilled water temperature range, a 2°F temperature increase implies 20% of the chiller capacity has been lost to heat gain in the piping system! The coil would have to be selected with only an 8°F chilled water temperature range. With the exception of extremely large piping systems, there is very little temperature increase in a properly designed and installed system.
2.0 Chilled Water Temperature Range
Increasing the chilled water temperature range reduces the required flow rate and consequently the pump and piping sizes. In some situations, the savings both in capital cost and operating cost can be very large. Increasing the chilled water temperature range while maintaining the same supply water temperature actually improves the chiller performance because the chiller log means temperature difference increases. It has just the opposite effect on the cooling coil where the LMTD decreases between the air and the chilled water. In some cases, it may be necessary to lower the supply water temperature to balance the chiller LMTD with the coil LMTD.
Table 3 provides suggested supply water temperatures for various ranges. The best balance of supply water temperature and range can only be found through annual energy analysis because every project is unique.
Products such as fan coils and unit ventilators have standardized coils that are designed to work with a 10 to 12°F chilled water range. When these products are used with this range of chilled water, they provide the sensible heat ratio and return water temperature generally required. When the range is increased, the coils may not provide the necessarily sensible heat ratio and return water temperature. It is recommended that for these products, the chilled water range stay close to industry standard conditions. Chilled water coils are designed for application-specific conditions so this is generally not an issue.
Condenser Water Temperature Range.
3.0 Condenser Water Temperature Range
Increasing the condenser water temperature range reduces the condenser water flow, which requires smaller pumps and piping. It also increases the required condenser pressure while improving the LMTD for the cooling tower. Increasing the condensing pressure on the chiller will result in a combination of increased chiller cost and reduced performance. Improving the cooling tower LMTD allows a smaller tower to be used, but the savings from this strategy will not generally offset the increased cost of the chiller.
In most cases, the overall design power requirement will go up. At full load conditions, the increased chiller power required to overcome the increased lift will more than offset the savings from the smaller cooling tower fan and condenser pump.
This will depend on the head requirement of the condenser pump. As the chilled water load decreases, the chiller and cooling tower work will reduce but the condenser pump work will remain the same. At some part-load operating point, the savings from the smaller condenser pump will offset the chiller penalty and for all operating points below this, the increased condenser range will save energy. Whether an increased condenser temperature range will save energy annually will depend on when the crossover point occurs (the pump motor size) and the chiller operating profile (whether the operating hours favour the chiller or the pump). This can only be found with annual energy analysis.
4.0 Temperature Range Trends
Changing the temperature ranges and supply temperatures requires careful analysis. The following are some points to consider:
The traditional AHRI operating conditions work very well for many buildings.
Unnecessary reduction of the chilled water supply temperature should be avoided as it increases chiller work.
When using standard products such as fan coils and unit ventilators, maintain the chilled water temperature range between 10 and 12°F where they are designed to operate.
Increasing the chilled water temperature range is a good way to reduce the capital and operating cost of a building, particularly if the pump head is large or the piping runs long.
With larger chilled water temperature ranges, it may be necessary to lower the supply water temperature to find a balance between coil and fan performance versus chiller performance.
If the chilled water supply temperature is reduced, consider oversizing the cooling tower to reduce the condenser water temperature and minimize the effect on the chiller.
Always take into account the actual design ambient dry-bulb and wet-bulb conditions when designing a chiller plant. If the location is arid, then lower the wet-bulb design as per ASHRAE design weather data and select both the cooling tower and chiller accordingly.
For very large chilled water ranges, use series chillers possibly with series counterflow condenser circuits to optimize chiller performance.
Increasing the condenser water range should only be considered for projects where the piping runs are long and the pump work high. When it is required, optimize the flow to the actual pipe size that is selected and select the chillers accordingly. Consider oversizing the cooling towers to minimize the effect on the chiller.
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