ASHRAE Journal - July 2011 - 20

occur, so tuning can be difficult. If the control loop is unstable, cold chilled water supply can be fed back into the return intermittently and cause chillers to cycle off due to low load or cold supply water temperatures. But if the loop is too slow, it may not respond quickly enough to sudden changes in flow (e.g., when a large number of air-handling units shut off at the same time), causing insufficient flow through the chillers, causing them to trip on low flow or low temperature. Complex control systems are prone to failure, so at some point in the life of the plant, one can expect the bypass control to fail. A failure of the bypass system can cause nuisance chiller trips, which generally require a manual reset. If an operator is not present to reset the chiller, the plant can be out of service for some time.

Central Chilled Water Plants Series
This series of articles will summarize the upcoming Self-Directed Learning (SDL) course called Fundamentals of Design and Control of Central Chilled Water Plants and the research that was performed to support its development. The series will include five segments: Chilled water distribution system selection. This article will discuss distribution system options, such as primary-secondary and primary-only pumping, and provide a simple application matrix to assist in selecting the best system for the most common applications. Condenser water distribution system selection. This article will discuss piping arrangements for chiller-condensers and cooling towers, including the use of variable speed condenser water pumps and water-side economizers. Pipe sizing and optimizing ΔT. This article will discuss how to size piping using life-cycle costs then how to use pipe sizing to drive the selection of chilled water and condenser water temperature differences (ΔTs). Chillers and cooling tower selection. This article will address how to select chillers using performance bids and how to select cooling tower type, control devices, tower efficiency, and wet-bulb approach. Optimized control sequences. The series will conclude with a discussion of how to optimally control chilled water plants, focusing on all-variable speed plants. The intent of the SDL (and these articles) is to provide simple yet accurate advice to help designers and operators of chilled water plants to optimize life-cycle costs without having to perform rigorous and expensive lifecycle cost analyses for every plant. In preparing the SDL, a significant amount of simulation, cost estimating, and life-cycle cost analysis was performed on the most common water-cooled plant configurations to determine how best to design and control them. The result is a set of improved design parameters and techniques that will provide much higher performing chilled water plants than common rules-of-thumb and standard practice.

2. Staging Control Complexities
When one or more chillers are operating and another chiller is started by abruptly opening its isolation valve (or starting its pump for dedicated pumps), flow through the operating chillers will abruptly drop. The reason for this is simple: flow is determined by the demand of the chilled water coils as controlled by their control valves. Starting another chiller will not create an increase in required flow, so flow will be split among the active machines. If this occurs suddenly, the drop in flow will cause operating chillers to trip. To stage the chillers without a trip, active chillers must first be temporarily unloaded (demand-limited or setpoint raised), then flow must be slowly increased through the new chiller by slowly opening its isolation valve. Then, all chillers can be allowed to ramp up to the required load together. During the staging sequence, chilled water temperatures will rise somewhat. This is seldom a problem in comfort applications, but may be an issue for some industrial applications. Given these considerations, primary-only systems are most appropriate for: • Plants with many chillers (more than three) and with fairly high base loads, as might be expected in an industrial or data center application. For these plants, the need for bypass is minimal or nil due to the high base loads, and flow fluctuations during staging are small due to the large number of chillers. • Plants where design engineers and future on-site operators understand the complexity of the controls and the need to maintain them. The primary-secondary system may be a better choice for buildings where fail-safe operation is essential or on-site operating staff is unsophisticated or nonexistent.

Primary – Distributed Secondary
For plants serving groups of large loads such as buildings in a college campus, terminals in an airport, etc., the primary-distributed secondary system (Figure 6) is usually the best solution. The secondary pumps at the central plant are deleted and variable speed pumps are added at each building. The building pumps are controlled by differential pressure sensors at the most remote coil in each building. Building pump heads are
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sized for the pressure drop of the loop from the plant, to the building, through the building’s coils, then back to the plant through the common leg. Therefore, each pump has a different head customized for the building. The advantages of this design compared to conventional primary-secondary and primary-secondary-tertiary systems include: • Overall pump horsepower is reduced. With the conventional system, secondary pump head must be sized for the most remote building (say 100 ft [299 kPa]) while the distributed building pumps close to the central plant can have much smaller heads (say 50 ft [150 kPa]).
ashrae.org July 2011

ASHRAE Journal - July 2011

Table of Contents for the Digital Edition of ASHRAE Journal - July 2011