By Chad Edmondson
Practically any commercial or institutional building has a certain amount of diversity within its cooling load, meaning that peak loads will never occur simultaneously in all sections or zones of a facility. By mapping out the individual load patterns of these sections, engineers can adjust the mechanical design to reduce the overall amount of installed cooling capacity. This means incorporating variable flow, which necessitates precise hydronic balancing.
To illustrate this point, consider this simple example of a central chilled water system at a college with four basic groups of buildings and identical peak loads:
Building Peak Load
Dorms 1000 tons
Cafeteria 1000 tons
Library 1000 tons
Gym 1000 tons
Clearly, the load pattern of these buildings will vary and at no time will there be concurrent peak loads in all four buildings. (Students can’t be in more than one place at a time!) In other words, there is diversity within the system. This gives the designer the opportunity to design the system so that the cooling water is directed only where it is needed.
Let’s say that the design engineer has done a complete cooling load calculation and has determined that the peak block load at any given time is 3000 tons. Block load is the instantaneous maximum heating and cooling load for a calculated point in time for the entire building, including all envelope and internal load components of the heating and cooling load calculation.
The engineer determines the peak block load based on the diversity factor that he or she has chosen for the system given the anticipated load patterns of the system. In our example, the engineer would have chosen a diversity factor of .75 because the diversity factor is the peak block load (3000 Tons) divided by the total connected load (4000 Tons).
Here are some very general rules of thumb for diversity in buildings:
- .85 for systems up to 25 tons
- .80 for systems from 25 tons to 100 tons
- .75 for systems larger than 100 tons.
If this particular system were designed in the old style with 3-way valves to provide constant flow through the chillers (Figure 1), peak block load would not matter because constant flow systems do not take advantage of diversity. You would have 2400 GPM of constant flow going to the dorms, cafeteria, library and gym at all times. And the system would require 4 chillers at 1000 tons each instead of just three.
However, we can get by with significantly less cooling capacity and less GPM by taking advantage of the diversity within the system and incorporating variable speed pumps. (Figure 2).
Notice that the system now includes variable speed pump controls and 2-way valves instead of 3-way valves. As a result we’ve trimmed most of the excess out of the system. We’re also doing the same job with less equipment:
- One less chiller, and 1000 fewer tons
- One less chiller pump
- One less cooling tower
- One less condenser water pump
- Reduced flow (7200 GPM vs. 9600 GPM)
- Smaller pipe main (18” vs. 20”)
Obviously there is a lot to be gained both in terms of equipment cost and efficiency, but balancing is more critical than ever. Why? Because although you have reduced the overall system flow, the peak flow requirements for each section have not changed. They were 2400 GPM before, and they are 2400 GPM now, so balancing must be carefully integrated into the system to assure that the maximum flow can be obtained if needed. In doing so we not only help ensure successful operation the system, but also meet the requirements of ASHRAE 90.1 for balancing.