Planning for Part Load
Variable-flow solutions for chilled-water systems address diversity in building loads
As occupancies change in buildings, so do cooling loads. If a chilled-water system is used, the control of chilled-water flow becomes important for occupant comfort and efficient operation. Consequently, focusing on the design-flow parameters to insure that the cooling capacity is adequate may result in a system that does not function efficiently at part load.
Not only must a chilled-water system be designed to operate effectively at part-load conditions, it must be able to recognize the existence of diversity in the building load. Diversity is defined as a chilled-water system's maximum load divided by the installed load, often called the block load . Thus, if a system has a maximum load of 800 tons and the installed load is 1,000 tons, its diversity is 80 percent. Diversity results from occupancy variation or varying physical conditions such as sun load, and cannot be assumed to be a constant for the total building.
Along with recognizing diversity, the system control must be configured to eliminate the contingency factors that exist in the design of system capacity and pump head. For example, a system may be designed to have a 20-percent increase in load within five years. Because it has been assumed that this load increase will cause an accompanying friction increase of 30 percent, the control arrangement must be such that these increases are eliminated from initial operation when the building is first occupied. A model system
The best way to demonstrate the procedures for accommodating diversity and occupancy variation is to use a model building with a variable-volume, chilled-water system that has proper control. Figure 1 depicts such a building that is taking chilled water from a central chilled-water plant and pumping it to 12 air-handling units (AHUs), each requiring 200 gallons per minute (gpm) at their design-flow conditions. The chilled-water system is divided into two principal loops—an east and west loop—each with six AHUs.
Before considering variable occupancy, assume that the building has a significant sun load that results in a diversity of 90 percent for sun-load variation, or 200 gpm for the AHUs on the sun side and 160 gpm for the units on the shade side. The total load is: (6 x 200) + (6 x 160), or 2,160 gpm. Using these loads, the system can be evaluated at full load and uniform occupancy. It must be emphasized that the uniform occupancy does not exist under normal conditions.
Figure 2 provides a hydraulic gradient for the east loop under full sun load; a similar hydraulic gradient could be drawn for the west loop with no sun load. The friction losses shown are: 8 feet to and from the central chilled-water plant; 8 feet through the pumping system fittings and valves; 10 feet to and from the pumping system to the first AHU in each loop; and 8 feet for the supply and return between any two AHUs. The losses between the pumping system and AHUs are for either loop when exposed to the sun load with the AHUs in that loop taking 200 gpm each.
In both Figures 1 and 2, a differential pressure of 30 feet is being maintained across the coils to overcome a loss of 13 feet in the coil, 2 feet in piping and 15 feet in the coil-control valve. This gives the control valve an authority of 50 percent, which is often needed for satisfactory control. It is important that this be emphasized now, because the only possible way for this chilled-water system to function properly is to have correctly designed control valves.
It should be noted that the setpoint of 30 feet for the differential pressure controller is a major part of the pump head. The question is how can this be reduced under part-load situations? The answer is that coil valve-position indicators or other physical parameters indicative of the system load can adjust the set point for the transmitters. Also, the set point for the differential pressure transmitters can be adjusted so that critical loads control the set point. When they cannot maintain desired conditions, the set point is increased. It should also be noted that the energy savings achieved by reducing the differential pressure set point at low loads on the system is offset by the low wire-to-shaft efficiency of the variable-speed drive and motor at minimum speeds.
To prevent the pumps from continually changing their speed, the differential pressure transmitters must provide a signal to the pump controller at least once every second. Faster rates of response, as high as 10 times per second, are needed on some applications. Parameters for part load
While the above discussion has been based entirely upon uniform occupancy in both loops of this system, it is important to note that, in reality, such a situation does not exist. Occupancy is variable—hourly, daily, weekly and annually. How does this affect the chilled-water system? Some loads, such as sun load, vary independently of the occupancy. Other loads vary directly with the occupancy.
In summation, the load change that is due to occupancy varies with the physical and operational characteristics of each building.
The following is an attempt to analyze the effect of variable occupancy on the flow and pump head required by a chilled-water system. The simplest way to do this is to have some coils operating at full load and others completely unloaded, and although such a design is practically impossible, it is necessary to assume this condition for the purposes of this discussion.
The east loop with the sun load is used, assuming that there will be no load on the west wing to clarify the friction losses. The friction losses are shown horizontally in the figures with the net pump head at any junction shown in the vertical position. No pipe sizes are shown, as the interest here is in variation of friction loss, not in design. The friction values shown are not as important as the variation due to the placement of the various loads.
The groups of coils demonstrate the effect of occupancy variation on the total pump head for the water system. The shaded coils in the figures are fully loaded, while the unshaded coils have no load upon them.
First, let the load in the east zone be located in the three coils farthest from the pumps (10, 11 and 12 in Figure 3). The pumping system flow is 600 gpm, and the pump head is now around 69 feet.
Next, let the load in this zone be located in the three coils nearest the pumps (7, 8 and 9 in Figure 4). The pump flow is the same, and the head is now around 35 feet.
There are three friction losses for the east loop, one at 100-percent load and two at 50-percent load in the loop. With uniform loading in all of the coils, friction losses are 76 feet, as shown in Figure 1. When only the three coils farthest from the pumps—10, 11 and 12 in Figure 3—are fully loaded, the loss drops to 63.4 feet. When the three coils nearest the pumps—7, 8 and 9 in Figure 4—are loaded, the loss drops to 33.6 feet. These figures can be compared to 42.8 feet with uniform loading at 50-percent load on all six coils.
One can conduct this procedure by adding and subtracting the number of coils fully loaded to develop the possible system head area for the east loop of this system (see Figure 5, page 46). Not all operations are as extreme as these examples, but the friction on the pumping system due to variable loads, including occupancy, can be at any point in this area. The actual system head area may not be as symmetrical as Figure 5; it may be uneven where there are no smooth curves outlining the shaded system head area. Recording the actual points of operation for simultaneous pump head and system flow will enable this actual area to be developed with software design tools. Lessons learned
What is learned from all this?
Firstly, variable occupancy can cause the load to shift to any part of the system. Therefore, the pumping system and its control must be designed to accommodate this.
Second, there is no means of manually balancing such a system. Balance valves should not be used on variable-volume, multiple-load systems such as this one. Note that coil number 7 had 70 feet of differential pressure when the east loop was fully loaded; it had a differential pressure of 61.8 feet in Figure 3 and 32 feet in Figure 4. There is not a single position for a manual balance valve.
Third, the coil-control valves must be designed to withstand the maximum differential pressure that can be imposed upon them by the pumps. Coil number 7 would require a valve capable of withstanding 70 feet of differential pressure while coil number 12 would never have any more than the differential pressure transmitter set point of 30 feet.
There is no need to go through all of this analysis to properly design pump-control and coil-control valves for every system. With the differential pressure transmitters located at the ends of the loops, as shown in Figure 1, the pump control will respond automatically to any change in the diversity or occupancy of the building. The pumps must have their speed respond to the variation in loads, and this is achieved by having the differential pressure transmitters relaying at least once per second.
Another concern is how to predict the pumping energy consumption for a prospective heating, ventilation and air-conditioning water system? It is almost impossible to do this accurately without an extensive evaluation of the loads that can exist on a specific building, particularly considering occupancy variation. Wrap up
The simplified shifts in load that were evaluated here are not as diverse as those that can occur in an actual building. It is important that the system designer understand the actual occupancy shift that can occur in a particular building and the effect that it can have on the heating or cooling system, particularly in regard to the pump performance.
An interesting example of occupancy variation is a large airport where arrival and departure of airplanes create sizeable swings in total occupancy. At the Denver International Airport, distributed pumping was used to reduce the occupancy effect. Each concourse has its own chilled-water pumping system that responds to the differential pressure transmitters located at the ends of the loops in each concourse.
This creates a sizeable reduction in pumping horsepower over the central chilled-water pumping system in the chiller plant. Although this reduced the energy consumption for the airport, the occupancy in each concourse still created the variation for the loops, as described above and illustrated in Figure 1.
In summation, on multiple building installations, the location of the pumping system or systems must take into consideration the occupancy shift from building to building. And then, the occupancy shift within each building must be studied to determine the actual head on the water system for that building.
In any case, it is always key for the design engineer to remember that shifts in diversity and occupancy are real factors that affect pumping-system performance and energy consumption.