Chilled Water

Chilled water typically has a supply temperature of between 42°F and 48°F. Historically, the return temperature was often chosen to be 10°F above the flow temperature. With the higher cost of fuel and the concern over energy usage, it is usually cost effective to design for a higher difference of 15°F or even 20°F. The higher return temperatures require larger coils, and create chal- lenges when high dehumidification is required.

On the other hand, doubling the temperature difference halves the volume flow, and, consequently, reduces the purchase cost of piping and pumps, as well as substantially reducing ongoing pumping power costs.

With a flow temperature in the range 42°F to 48°F, the piping must be insu- lated to reduce heat gain and avoid condensation. The insulation requires a moisture barrier on the outside to prevent condensation on the pipe.

Chillers, the refrigeration equipment used to produce chilled water, mostly use a direct expansion evaporator. Therefore, the flow must be maintained fairly constant to prevent the possibility of freezing the water. The chiller

Hydronic System Architecture 127

Figure 9.9 Pumps in Parallel

requires constant flow but it would be both convenient and economical to have variable flow to the loads. To achieve this, the chiller and loads can be hydraulically “decoupled.” Decoupled, in this context, means that the flows in the chiller circuit do not influence flows in the load circuit. Conversely, changes in the flows in the load circuit do not affect the chiller circuit.

A diagram of two chillers and loads is shown in Figure 9.11. The two chillers are piped in parallel in their own independent pipe loop, shown bold in the Figure. The chiller loop can run even if the distribution pumps are off. Similarly, the distribution loop can run with the chiller pumps off. The short section of shared pipe allows both loops to operate independently of each other, decoupled. Each chiller has a pump that runs when the chiller runs, producing a chiller- circuit flow of 50% or 100%. The flow in the cooling-loads circuit is dependent on the distribution pumps and whether the valves are fully open or throttling (reducing) the flow. If the chiller flow is higher than the coil circuit, water will flow through the short common section of pipe as the excess chiller water flows round and round the chiller loop. If the chiller flow is less than the coil circuit flow, than some coil return water will flow through the short common section of pipe and mix with the chilled water. When this happens, a flow or temperature sensor will detect it and start another chiller.

The loads in Figures 9.11 and 9.12 are shown as having two way valves which have no flow when they are closed. If all the valves were to close, the pump would be pumping against a closed circuit. To avoid problems occur- ring when this happens, a bypass valve is shown across the end of each branch circuit to allow a minimum flow under all conditions.

The arrangement in Figure 9.11, with distribution pumps serving all loads, requires these pumps to run regardless of the load. On projects where sections of load may be shut down while others are running, a “distributed” pumping arrangement may be more efficient. In Figure 9.12each secondary loop has its own pump, which is sized to deal with its own loop resistance and the main loop resistance. This system allows pumps 1, 2, and 3 to be run independently, when necessary, to serve their own loads.

The development of economical and sophisticated computer control and affordable variable speed drives, now enables designers to organize piping and

pumping systems that really match need to power, compared to the historical sit- uation where the system used full pump power whenever the system was “on.”

9.6 Condenser Water

Condenser water is water that flows through the condenser of a chiller to cool the refrigerant. Condenser water from a chiller typically leaves the chiller at 95°F and returns from the cooling towerat 85°F or cooler. The cooling tower is a device that is used for evaporative cooling of water.

In Figure 9.13the hot, 95°F, water from the chiller condenser flows in at the top. It is then sprayed, or dripped, over fill, before collecting in the tray at the

Hydronic System Architecture 129

Figure 9.12 Distributed Secondary Pumping

bottom. Air enters the lower part of the tower and rises through the tower, evaporating moisture and being cooled in the process, before exiting at the top. We will consider cooling towers in more detail in the next chapter, but the tower has a hydraulic characteristic that we will cover here. The water has two open surfaces, the one at the top sprays and the other at the sump surface. This is an open-water system. An open-water system is one with two, or more open water surfaces. Aclosed-water systemhas only one water surface.

Figure 9.14 shows an outline elevation of the complete cooling tower and chiller condenser water circuit. The water loop has two water surfaces, one at the top water sprays and one below at the sump water surface. When the pump is “off,” the water will drain down to an equal level in the tower sump and in the pipe riser, as indicated by the horizontal dotted line in Figure 9.14. When the pump starts, it first has to lift the water up the vertical pipe before it can circu- late it. The distance that the pump has to lift the water is called the “static lift.” Once running, the pump has to provide the power to overcome both the static lift and the head, to overcome friction, to maintain the water flow.

Figure 9.15shows a closed water circuit. It is shown with one water surface open to the atmosphere. Whether the pump runs or not, the water level stays constant. When the pump starts, it only has to overcome friction to establish and maintain the water flow. When the pump stops, the flow stops, but there is no change in the water level in the tank. The open surface is required to allow for expansion and contraction as the water temperature changes during operation. In larger systems and most North American systems, the one open water surface is in a closed tank of compressed air rather than open to atmos- phere, as is common in other parts of the world.

The cooling tower provides maintenance challenges. It contains warm water and dust, so it easily supports the multiplication of the potentially lethal bacte- ria, legionella.

We will return to cooling towers, their design, interconnection and operation when we discus central plants in the next chapter.

The Next Step

This chapter has covered hydronics architecture, specifically the piping systems for steam, hot water, chilled water and condenser water. In Chapter 10 we are going to consider the central plant boilers, chillers and cooling towers that produce the sources of steam and water at various temperatures.

Summary

In this chapter, we covered hydronics systems, systems involving the flow of steam or water to transfer heat or cooling from one place to another.