“Central plant” is a general term for the building or space accommodating the major pieces of HVAC equipment. The equipment there typically consists of a chiller, boiler, and hot-water heater with their associated pumps and controls. In addition, the chiller has a cooling tower operating on its own set of pumps. These items comprise the three major systems in a building: space cooling, space heating, and hot-water heating. Each system has its own piping, fittings, and pumps. Each system also has energy-conservation projects peculiar to its purpose and method of use, although there are some common denominators.
Each of the major mechanical systems circulates water through a piping loop. The building cooling and heating systems are closed loops, since the same volume of water circulates constantly through the loop. The condenser water circuit is an open loop. As such, the water is pumped from the condenser side of the chiller to the cooling tower, where it’s discharged over the honeycombed panels of the tower that are subjected to a strong forced airflow to maximize evaporative cooling. The hot-water circuit is also an open loop, as the water circulates around a closed loop, from which it is discharged at the point of use.
A key aspect of a fluid system is the existence of impurities, and the system’s level of tolerance for them. Dissolved solids make the water hard and suspended solids cloud the fluid. Each type of contamination represents a different kind of threat to the various fluid circuits.
Fluid contamination in the chilled-water loop is the simplest to understand. Water circulates through the chiller, then through the building loop to finned coils in each air-handling unit. There, the water absorbs heat from the air forced across the coils and returns to the chiller to be cooled down again.
This system is entirely closed, so there is little opportunity for leakage or other losses. The only threat is rusting or pitting of the steel pipe, because oxidation reduces the structural strength of the pipe. It also increases the friction of fluid flow, making the pump use more energy to maintain sufficient flow at the coils for proper space conditioning. Oxidation also introduces rust particles to the fluid stream. These, in turn, can damage the pump impeller, jam valve components, and pit the inside of the pipe.
Chemicals and lubricating compounds can be added to the circulating fluid to minimize oxidation. The fluid can be filtered, to eliminate any large particles before the more sensitive components of the loop are damaged. If dissolved solids become a mitigating problem, then the entire fluid volume can be replaced with clean, treated water. Many buildings drain their systems every year anyway at the beginning of the heating season. Those that require cooling throughout the year, though, need another means to maintain water quality.
The other building fluid systems must cope with the above circumstances, plus a few potentially more debilitating problems. Dissolved solids can precipitate out of solution, such as when water passes through the tubes in a boiler. Over time, these solid particles can clog boiler tubes. This restricts the fluid flow, which increases the static pressure, which must be overcome by the circulating pump. It also acts as an insulating layer, reducing the heat transferred to the circulating water. The result is that more heat is wasted up the flue, since it cannot be absorbed by water passing through the tubes.
This process occurs in both the space-heating system and the domestic hot-water system. The former is a closed system, so it is economical to add chemicals to the water to reduce the dissolved solids and to maintain a neutral pH. The domestic hot-water system, however, is an open loop. It has such a high flow rate that the best most facilities can do is to install a water softener to reduce a portion of the dissolved solids. Also, the piping can be oversized, and the fittings, valves, and boiler tubes can be designed for convenient inspection, cleaning, or replacement when the scaling causes problems.
In a boiler, where steam is constantly lost, this process is even more exacerbated. The steam cannot transport dissolved solids, so when it leaves the loop the concentration of solids in the rest of the circulating fluid increases. As a result, chemicals must be added at regular intervals and/or a portion of the fluid must be dumped periodically and replaced with clean water. This purging is called boiler blowdown. It rids the system of heavy, thick sludge. It also decreases the overall concentration of dissolved materials. Since fewer chemical additives are needed for boiler blowdown, it might seem thriftier, until you realize that the fluid dumped is high-pressure steam whose embodied thermal energy is wasted.
The last fluid system, the condenser water loop for the chiller, has all of these problems, plus even more. Since the fluid is exposed to a forced airflow at the cooling tower, it can absorb atmospheric particles, dust, and smoke. These microscopic solids can plug water-flow passages in the chiller and cooling tower, foul temperature and pressure sensors in the piping, cavitate pump impellers, and cause extremes of pH. In addition, the aeration that happens at the cooling tower causes a high oxygen content in the water which, combined with the warm discharge temperature from the chiller, is conducive to bacterial growth.
All of these difficulties with water composition vary from one location to another. They’re dependent upon air quality, water quality, and the condition of the piping and other equipment. Whatever the situation, though, chemical treatment is a critical maintenance activity, and the avoidance of unnecessary water losses is paramount, especially where steam or hot water is involved.
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Excerpt from Energy Conservation Projects.