District Energy - 2005 Third Quarter

adoption in the early 1960s when government buildings in Washington, D.C., embraced them for creating cold water for perimeter induction cooling. Induction systems supply cold primary air to the space, requiring colder water from the chiller. Chillers in those days had a coefficient of performance (COP) of about

4.0, with high-flow (velocity) smooth-bore tubes, low tube-counts and one-pass evaporators to reduce pressure drop.

In the 1970s, variable-air-volume (VAV) systems made the colder chilled water used for induction systems unnecessary.

Why Series? Given the chiller’s relatively low efficiency

Multiple-stage or series chillers pro- by today’s standards, it made sense to vide rigorous, stable cooling at extreme raise temperatures. conditions. Series chillers are standard VAV systems were widely adopted chillers that are piped or lined up in a because they save energy and adapt to series, which allows the system to use less unknown cooling loads. VAV systems are energy to cool. still the most popular choice for delivering

Figure 1. Series counterflow chiller arrangement equalizes lift performed by each compressor, minimizing the energy needed to create high lift.

gal, screw and absorption chillers; and air-cooled and water-cooled chillers. The district cooling business model is also key – overall chiller plant efficiency goes directly to the bottom line.

More and more chiller plants are selecting low-flow, low-temperature, yet highly efficient chillers (see sidebar). Because pump energy is proportional to the cube of the flow, even incremental flow reductions quickly result in net positive cash flow. But when flow goes down, temperature must also, favoring series chillers.

Pressure Drop

A typical maximum acceptable chiller pressure drop is 25 ft of water. Even after adding more tubes to reduce pressure drop, two chillers in series might use twice that, because each chiller has twice the amount of water going through it. In traditional primary-secondary systems, constant flow through the chillers equals constant pressure drop and a constant pump energy.

Today, variable-primary systems send variable amounts of water flow through the chillers to limit the effects of pressure drop at many load conditions. Variable-primary systems are made possible by the latest chiller technology and control advancements. Series evaporators reduce the need for a bypass in variable-primary systems because the higher initial water flow allows for higher flow reductions before reaching the minimum flow for the chiller.

Courtesy Trane Commercial Systems.

When chillers are lined up in parallel, each individual chiller must provide the coldest water required for the entire system. In series, each subsequent chiller in the process can operate more efficiently and provide colder water. It also uses less energy for high ambient wet-bulb conditions, which are common in the Middle East and China.

Common reasons why some designers do not go with series are fear of something ‘new,’ lack of redundancy and higher water-pressure drop. All of these reasons are well-understood, and current chiller designs compensate appropriately.

conditioned air; however, series chillers offer additional benefits because chillers have almost doubled in energy efficiency.

History of Series Chillers Series chillers saw widespread

Redundancy

System designers are finding that large chiller plants can be more adaptive and efficient by installing multiple chillers rather than one or two large, field-erected chillers. In plants with more chillers, redundancy is easily created through parallel banks of upstream and downstream chillers. Different amounts of upstream and downstream chillers can meet the load, so if one chiller is being serviced, its duty can be spread out to many other chillers.

Chiller Technology and Its
Impact on Design

The most efficient centrifugal chillers today have COP of more than 7.0 – which is more than 75 percent higher than chillers used in the first series chiller plants.

Chiller technology has had a sizeable impact on the design of chilled-water plants. The chiller’s responsive control and multiple-stage stability allows for more versatile designs. One example is variable flow through standard centrifugal chillers, a design discouraged by manufacturers just a few years ago.

Today’s advanced controls offer features that incorporate improvements to chiller capabilities and variable speed pumps. Improved tube designs and extensive testing in manufacturers’ testing labs have cut minimum water velocities in half, leading to better turndown and higher pump energy savings.

Chiller efficiency is dependent upon several variables – two of them are capacity (tons) and lift (chiller internal differential temperature). Multiple-stage centrifugal chillers have the ability to create high lift, which is roughly equivalent to the difference between the leaving condenser and leaving evaporator temperatures.