Process Applications in HVAC
Process applications can place unique demands on chiller plants. All parties involved in a process application should be fully aware that it is a process and not an HVAC application to avoid any confusion.
1.0 Process Load Profiles
Process loads can be broken down into specialized environments and chilled water requirements for the actual processes. Specialized environments such as low relative humidity (RH) for pharmaceutical or confectionery manufacturers are an extension of conventional HVAC design.
Chilled water for processes and equipment can be quite a bit different from HVAC design. Remember that process loads generally have very little to do with ambient conditions. It is quite possible that the process will be operating at 5% on the hottest day of the year and 100% on the coldest day. It is extremely important to gather as much information about the process, the load profile the operating conditions, etc., as possible. Adding a process load to an HVAC chiller plant without this consideration can lead to very poor performance.
The designer should gather at least the following information:
The process design load. Is it constant? Is it stepped or a batch load?
Are there multiple process loads with different needs? Do they have to be handled individually or can a common system serve both?
The operating hours. Is it every hour of the year? Is it only in the summer? This will lead to a discussion about redundancy. In HVAC design, redundancy often means there are at least two pieces of equipment but not necessarily enough capacity to meet the design load if one of them should fail. In process applications, redundancy usually means 100% backup so no production time is lost.
The critical nature of the process. Can the customer live without chilled water? If so, how long? All chiller plant equipment must be serviced at some point offline. How will this be accommodated?
Will the chiller plant service HVAC loads as well? If so, the combination of the two load profiles will need to be considered.
1.1 Constant Load Profiles
Constant load profiles have very little change for long periods of time. The goal here is to optimize the chiller plant for full load performance.
1.2 Stepped Load Profiles
Stepped profiles are most common. An example of a stepped process is cooling moulds for injection moulding. If there are only two moulds, then shutting down one line is an instant 50% reduction in chiller plant load. The quick changes in load must be accounted for. The chiller plant should be optimized to perform well at the various “plateaus” in a stepped load profile.
1.3 Batch Load Profiles
Batched operations, such as a bakery, require a relatively sudden amount of chilled water and then nothing for an extended period of time. This profile lends itself to some form of chilled water storage and charging in anticipation of the load. The load size and the intervals are critical to proper operation.
2.0 Condenser Relief
As mentioned earlier, there is typically no correlation between process load and ambient conditions. Most chiller ratings and designs are based on HVAC load profiles which are tied to the ambient conditions. This is very true for condenser relief where AHRI 550/590 allows a condenser relief profile based on HVAC design. For example, consider a centrifugal chiller used in a process application. The processing load drops to 25% on the hottest day of the year. It would not be expected in an HVAC load to ever operate at 25% on a design day. The chiller may not even be able to meet the lift requirement at these conditions. Chillers used for the process must be rated at process conditions.
As the load on a cooling tower goes from 100% to 0%, the leaving condenser water approach will go from the design approach to a 0°F approach. For example, if the supply water is 85°F with an ambient wet-bulb of 78°F, there is a 7°F approach. This can be used to estimate the correct condenser relief for a process chiller on a design day. For instance, in the example given above, the entering condenser water at 25% load and AHRI 550/590 conditions would be 65°F. Using the above relationship for cooling towers, the actual entering condenser water temperature would be 79.75°F. Process chillers should be selected where possible to operate down to minimum capacity with the condenser water temperature being the same as the design wet-bulb. If the stable operation for the chiller is exceeded, then a hot gas bypass should be added.
3.0 Winter Design
Most process chillers operate year-round. In colder climates, winter design must be considered. The equipment manufacturer should be involved in assessing the safe, reliable operation of the chiller in subfreezing conditions.
The very high operating hours provide an excellent opportunity for using the cooling towers and a heat exchanger to directly cool the processing load in a manner similar to waterside free cooling (Refer to Waterside Free Cooling)
4.0 Chilled Water Volume
Process loads with sudden changes in load are a challenge for a chiller. The best method to deal with them is to have a large enough flywheel effect in the chilled water system to limit the rate of change seen by the chiller. There may be enough chilled water volume in the system to provide the necessary flywheel effect. If not, then a tank may be required. Different chillers have different limits for the rate of change and thus require different system volumes. Refer to Minimum Chilled Water Volume and chiller product catalogues for details.
5.0 Temperatures and Ranges
Chilled water supply temperature and chilled water ranges must be reviewed and based on the needs of the processes served. It should be first surmised what chilled water temperatures and ranges are required and whether a single supply water temperature will work. The use of a tertiary pump can allow for different temperatures but the chiller plant will have to provide all the chilled water at the lowest required temperature.
This will lower the chiller efficiency for the entire plant. It may be advantageous to have medium and low temperature chilled water systems.
In many process applications such as injection moulding, avoiding condensation is important. Confirming that the chilled water temperature is above the space dew point is very important. The warmer the chilled water, the more efficient the chillers. There are processes that require chilled water warmer than the chiller is rated to provide. This can be resolved with mixing valves. Temperature ranges can also vary a lot. For instance, a process may require a 60°F supply of water and a 30°F temperature range. These conditions are beyond the range of most chillers so the system design must accommodate them.
Figure 70 shows a typical piping arrangement to deal with high supply water temperature, large temperature range and small chilled water volume. This arrangement allows the chiller to operate at optimized conditions while meeting the requirements of the process. The storage tank provides the necessary buffer to limit the rate of change.