Variable-flow chilled water systems

Many installed air conditioners use chilled water system and by installing ABB UK ACH550-01 inverter drives maintain higher comfort zones and reduce operating costs.

Chilled water supply temperatures typically range from 5 – 10°C and the return water temperature is 5 – 10°C higher. This temperature increase resulting from the air conditioning load is the key parameter for the variable flow system, and once selected all equipment must be designed to operate within that range. This means the chillers, air handling unit coils, and the control valves.

Chilled water plants are generally one of two types: variable flow or constant flow. To reduce installation and operation costs, it is best to vary the chilled water flow throughout the building. The main circulating pump must be a variable flow pump. Multiple pumps can be staged on, but ABB  ACH550 variable speed AC drives are commonly used.

Operation

In the illustration, space cooling of several rooms has chilled water circulating through a heat exchanger and thermostat valve in each room. The return water is circulated by a chiller water pump through the chiller evaporator (3). The water temperature is reduced by 5 to 10°C in the evaporator and supplied back to the rooms. The variable water circulation is achieved by the ACH550 drive, and the control parameter is the differential water pressure in the most distant room in order to keep the pressure high enough for the thermostat valves. The differential pressure is measured by the transmitter (4).

Creating the Comfort Zone

The use of variable speed ACH550 drives improves the controllability of the chilled water system, making it easy to maintain the Comfort Zone.  Although initial investment costs may be higher, the energy savings at the motors mean that an investment pay-back time of 0.2 – 3 years.

In addition to savings in electrical energy ABB drives provide fast and accurate control for maintaining comfort zone conditions and the desired air quality.  They also eliminate water hammer or hydraulic shock due to soft starting and stopping.  The use of only one pump also helps reduce consumption of electrical energy.

For power ratings up to 355kW contact us for information on the ACH550-02 Series

Open ABB application note here

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