Chilled water cooling load diversity

“What is the diversity ratio of the sum of the individual building peak load demands system when compared to the peak demand of the entire chilled water system plant hour by hour?”

Without the answer to this question, central chilled water plant performance becomes inefficient? This is because:

1. The initial central chilled water plan and its future equipment and distribution are based on estimates of future growth that may or may not occur in the years to come.
2. Each design engineer will use their conservative assumptions to provide sufficient cooling capacity, chilled water flow, and system pressure drops to accommodate their building design, and this information will be fed back to the central plant manager.
3. The design engineer will obtain central plant operating data for each building, but their focus will be on their own project. As a result, design engineers will continue to include their own conservative assumptions and safety factors for their projects, e.g., chilled water peak-demand tons (12,000 Btuh), pump head, and gallons per minute (gpm).
4. The central plant peak chilled water demand will be known, but the time of day as well as the individual building cooling peaks will most likely not be documented.
   

The overall central plant expansion topic will become a concern at some point in time, as new buildings are added to the central plant total capacity. The new building chilled water design parameters, safety factors, etc., added one new building after another, not considering previous over-designed parameters that influence central plant equipment operating inefficiencies — analogous to a snowball rolling downhill, growing larger and larger. The understanding of central plant capacity, redundancy, and the need to expand will become a mystery. A hydraulic fluid analysis of the central plant and distribution can prevent central plant operation deficiencies as each building is brought online.

Seldom will the central plant management maintain the hourly peak chilled water load requirements. As a result, the design engineer’s new building load requirement (say 460 tons) will be added to the total plant peak load (say 2,000 tons) without the knowledge of existing building-by-building, time-of-day database peaks. The upshot of a central plant peaking at 3 p.m. along with the new building load peak of 460 tons at 12 p.m. will result in a new, inaccurate total of 2,460 tons peak capacity, ignoring the question, “what is the new building chilled water central plant load at 3 p.m.”? This would lead to a discussion that the chilled water management should have peak central plant hour-by-hour capacity and its associated diversity factor versus connected load.

Without acknowledging the new building #6 peak load occurrence versus central plant peak load occurrence, the facility’s central plant manager and the design team will conclude there is a need to accommodate this new 460 tons when, at 3 p.m. the new #6 building chilled water load is estimated at 300 tons.

The campus-wide missing data point is time-of-peak demand. Here is a simplistic analysis using a five-building campus:

• Building #1 installed load is 400 tons and peak at 1 p.m.
• Building #2 installed load is 500 tons and peak at 3 p.m.
• Building #3 installed load is 600 tons and peak at 12 p.m.
• Building #4 installed load is 300 tons and peak at 2 p.m.
• Building #5 installed load is 400 tons and peak at 1 p.m.

The assumed total chilled water demand, minus the peak time-of-day data, would be 2,200 tons. Without a central plant campus-wide calculation and documentation of the time-of-day these peaks occur, the true total plant demand will not be known, and a new central plant capacity availability will be assumed.

As one can see, the existing five buildings have different peak cooling times. Just like a designer estimating air conditioning cfm for each of the individual room calculations in a new building, e.g., the cooling capacity for an east-exposure room may be 200 cfm at 8:30 a.m.. This same room won’t need 200 cfm cooling capacity when a south exposure room peaks at 400 cfm at 1 p.m. As a result, it’s standard HVAC engineering to calculate the individual room maximum cooling requirements and then do a “block load” to determine the building’s peak demand. This provides the “diversity factor,” e.g., 80% of the total individual room peak cfm’s. This is why a building with constant-flow central air handling may be 10,000 cfm, providing the constant/same supply air hour by hour, whereas a variable volume air-handling unit will provide only the needed supply air hour-by-hour based on the diversity factor, e.g., 8.000 cfm. This same methodology applies to individual building peaks versus the campus diversity factor for the overall campus “block load,” e.g., 60% diversity considering each building’s hours of occupancy.

Design engineers are familiar with the maximum demand for the central plant, but the overall campus block load and time of day will most likely not be known. For my discussion, I will say the central plant overall peak occurs at 3:30 p.m. with the following time-of-day demands:

• Building #1 is unoccupied with a minimum chilled water load of 75 tons at 3:30 p.m.
• Building #2 is partially occupied with a chilled water load of 350 tons at 3:30 p.m.
• Building #3 is partially occupied with a chilled water load of 450 tons at 3:30 p.m.
• Building #4 is partially occupied with a chilled water load of 200 tons at 3:30 p.m.
• Building #5 is partially occupied with a chilled water load of 200 tons at 3:30 p.m.

The resulting peak chilled water demand is 1,275 tons with a central plant diversity factor of 58%.

To the best of my knowledge, there is no organization that tracks campus central plants to catalog chilled water site system diversity by application, e.g., educational campus, industrial multibuilding site, or a health care multibuilding complex. An engineer can find budget values for Btuh per square foot (and Btuh per square foot per year) for an educational building, office building, hospital, etc.

An engineer can find budget values for various application stating cfm per square foot, square foot per ton, etc. But, when it comes to designing and continuously expanding central chilled water plants, there is no historical data to support a diversity factor for monitoring and maintaining the optimum diversity factor to know:

1. When does a certain central plant truly peak, at what time, and what are the other time-of-day peak loads?
2. At the plant’s true peak hour, what is the plant’s actual load versus installed/connected load?
3. How much additional capacity is truly available based on the central plant’s peak time of day?

When master planning and needing to know (1) central plant installed capacity, (2) standby capacity, and (3) peak load capacity based on the time-of-day capacities these categories are critical. To analyze, one must start with a hydraulic modeling of the plant (last month’s Tomorrow’s Environment discussion) to identify the system existing conditions, identifying inefficiencies, and to also establish time-of-day peak capacities before manipulating building-by-building installed load, peak loads, and future loads of central plants. The results will be: peak load demand, time of peak load demand, and availability to provide chilled water to support the next building program demand.