Improving the Efficiency of Chiller Plant
New York city has many chiller plants and most of them are operating inefficiently.
For the past 30 or more years most central chilled water plants have used primary-secondary pumping arrangements. The primary system includes all equipment to the plant side and the secondary system includes all equipment to the load. The majority of these systems have the chillers staged from flow. As the flow in the secondary starts to exceed the flow in the primary, an additional primary pump and chiller is staged on. As the flow in the secondary decreases, so that one less chiller can handle the reduced flow, a pump and chiller is staged off. This method of control assumes that the load is proportional to flow and that the dT between the chilled water supply and return is constant and at the design condition.
These assumptions are generally wrong. Systems rarely operate at design dT because of coils that are dirty, inadequate coil control valves, thermostats that are set wrong, excessive temperature drops in the supply and return mains, etc. Therefore, flow is not a reliable indication of load. As a result, the vast majority of central chilled water plants operate inefficiently with lower than design dT and with more partially loaded chillers on-line than would be necessary if the chillers were properly staged.
Traditional Chiller Plant Design
We will discuss traditional 1600 ton chiller plant operation in different scenarios.
The typical primary/secondary system is fully loaded at the outdoor design conditions of 95 degF dry bulb and 78 degF wet bulb. The system is operating as designed with 6,000 gpm supply water and 57 O F return water chillers and companion pumps are handling 2,000 gpm. The fourth standby pump is off.
In second scenario the load is only 600 tons. The dT is 6 deg F, which is typical of many systems designed to operate at a higher dT. Two pumps and chillers are on-line, with each chiller inefficiently loaded to 300 tons; 4,000 gpm is circulated in the primary, 1,600 gpm through the bypass and 2,400 gpm in the secondary. If the system were properly staged and piped, one chiller could easily handle the 600 ton load.
In third scenario the load has increased to 1500 tons is circulating in the primary and secondary systems with no flow in the bypass. Three inefficiently loaded chillers and three pumps are on-line. Two chillers could handle the load.
To satisfy the 1600-ton load in fourth scenario, 6400 gpm is required. The flow in the secondary loop exceeds the primary flow of 6,000 gpm and return water through the bypass starts to mix with the supply water. This raises the supply water temperature and the air-handling unit control valves start to open, thus increasing both the secondary and bypass flow. Temperature control in the conditioned spaces is lost, even though only about half of the central plant capacity is used. When chillers are staged from flow and are controlled at the design leaving chilled water temperature, the approximate maximum system capacity is:
(Operating dT) x (Design Capacity)/ Design dT
Very often facilities with operating conditions such as these will add an additional chiller and pump even though the existing plant would have sufficient capacity if the chillers were piped and staged properly. Adding more equipment is not the solution. The secondary pump and piping is not designed for the additional flow.
The capacity of a chiller, using hydrocarbon-based refrigerant, increases from 0.5 0 to I .5 0 tur every degree decrease in entering condenser water temperature. However, the increase in chiller capacity at a lower kW/ton cannot be used when chillers are staged from flow. Whenever the flow exceeds the design flow of the chiller in the typical primary/secondary system, the next machine is started on even though the operating machine(s) are not fully loaded. The maximum capacity of any machine never goes above the 100 % capacity – Most chillers are rated at 85 0 F (29 0 C) entering condenser water temperature, a condition that may exist less than 2 % of the year.
Although the remedy is relatively simple and inexpensive, operators are hesitant to change, thinking, “That was the way the system was designed to operate.” Initially everything possible must be done to correct the issues contributing to the low dT. Then change the way the chillers are staged on and off and install a check valve in the bypass line to prevent return water from mixing with supply water. The check valve and bypass line should be sized to handle the minimum flow of the largest chiller. A non-slamming check valve with a manual opening device is generally used. A temperature sensor in the secondary supply water, set low enough to satisfy the load requiring the coldest water, will stage the chillers on.
Getting maximum capacity from your chiller plant is directly related the operating dT. Making simple modifications to the existing chiller plant will improve the plant capacities.
If you want to increase your chiller plant efficiency, contact us. We will analyze and provide you options.
YEC Engineering has designed many chiller plant projects in NYC and other states.
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Baltu Yorkos, PE