Saturday, 20 February 2016

Primary and Secondary Chilled Water Pumps and their operation

Primary/Secondary Chilled Water Pumps

The secondary chilled water pump (SCHWP) sets will be controlled through VFD drives to maintain the desired system differential pressure. A minimum of 3 differential pressure transducers will be connected to the DDC controllers of the chilled water system and these shall be used for pump speed control. The differential pressure transducers shall be placed at approximately 2/3 distance on all main sub-branches and on the index run of the Chilled Water network.

The chilled water pump arrangement shall operate as 2 No duty, 1 No standby. Each pump shall be of variable volume type driven via a variable frequency drive, controlled via differential pressure measurement (the sensors shall be located approximately 2/3 of the index circuit) and the operation and controls shall be by the BMS. All cooling equipment shall be provided with 2-port pressure independent control valves for the control of chilled water flow.

When the pumps are signaled to start, the lead pump shall start first and a frequency inverter shall vary the speed of this duty pump depending on the differential pressure in the pipe work network. Differential Pressure in the supply and return chilled water pipe work shall be installed in the index run of each circuit approximately two thirds of the hydraulic distance from the pumps. The exact location of the pressure sensors shall be finalised in accordance with the recommendation of the controls manufacturer. When the chilled water 2-port valves on each AHU/FCU start to open due to increasing load, the differential pressure detector shall send a signal to the BMS, which in turn shall control the VFD on the CHW pumps to maintain the set differential pressure in the pipe work, thus increasing the CHW flow in the circuit.

In the event the lead pump reaches 90% of its maximum speed, its speed increase shall be arrested and the 2nd pump shall start and its speed shall increase until the differential pressure set point is met. Once met the lead pump’s speed shall reduce and the lag pump’s speed shall increase until both pumps have the same speed and are maintaining the differential pressure. From then on the speed of both pumps shall be the same.

When the chilled water demand has reduced to the extent that the duty pumps are operating at less than 40% (say) of their capacity, one of the duty pumps shall be switched off automatically and the other pump shall increase in speed to maintain the set pressure.

The above description is for two chilled water pumps operating in sequence. For three pumps, the sequence is similar except the changeover duty should be matched accordingly.

There shall be water flow proving differential pressure switch across each of the pumps. In the event there is no water flow detected across the pump or an inverter alarm after a pre­determined time delay after the pump has started, or during the operation of the pump, the BMS shall changeover to the stand-by pump and annunciate an alarm.

The BMS shall also monitor the MCC Trip Alarm, Auto Status, VFD Command, Feedback and Pump run status via DPS.

The actual duties at which lead/lag pumps are energised or de-energised shall be determined during the commissioning stage by the controls specialist.

The position of the HOA switches shall be monitored by the BMS and an alarm shall be annunciated on the BMS if any of the switches are not in the normal operating position. Chilled water pump ‘Run’ and ‘Trip’ indication lights shall be provided on the MCC for each pump. Hours run meters shall be provided for all chilled water pumps at the respective MCC.

A graphical representation of the plant will be produced with all set points, alarms and time schedules displayed with simple mouse clicks. Access to the graphic will be through a system of site plans, plantrooms and systems.

All values are to be historically recorded at controller level so that locally any laptop or Portable operator’s terminal may retrieve the data as well as the network BMS Supervisor.


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HVAC is the technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the field's abbreviation as HVAC&R or HVACR, (heating,ventilating and air-conditioning & Refrigeration) or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is important in the design of medium to large industrial and office buildings such as skyscrapers, onboard vessels, and in marine environments such as aquariums, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors. Ventilating or ventilation (the V in HVAC) is the process of "exchanging" or replacing air in any space to provide high indoor air quality which involves temperature control, oxygen replenishment, and removal of moisture, odors, smoke, heat, dust, airborne bacteria, and carbon dioxide. Ventilation removes unpleasant smells and excessive moisture, introduces outside air, keeps interior building air circulating, and prevents stagnation of the interior air. Ventilation includes both the exchange of air to the outside as well as circulation of air within the building. It is one of the most important factors for maintaining acceptable indoor air quality in buildings. Methods for ventilating a building may be divided into mechanical/forced and natural types.