In this module we will learn about pump head and delta T. We will also learn about chilled water flow.
Recall that the pump circulates chilled water from the chiller system and returns it to the chiller system. The pump should circulate the chilled water up to the total height of the building. The maximum height that the pump can circulate is the pump head.
The pump head is the height to which the pump can lift the water directly. It is measured in metres or feet. Pump head is useful because it tells the pump capacity to do a job.
If we want to pump cold water to a building height of 50 metres, but the pump has a head of 30 metres, the cold water will not circulate. We have to choose a pump with more than 50m head or use two pumps with 30m head.
If we choose a larger discharge head than required, it will consume more energy. It can even damage the chilled water pipes.
Water Cooled Screw Chiller
Delta T is the most common term used in the HVAC industry. It indicates the temperature difference between two points. For example, if water enters the evaporator at a temperature of 12°C and leaves the chiller system at 7°C, the Delta T is 5°C.
For example, the cooling system is designed for a delta T of 6°. During chiller operation, water enters the evaporator at 13°C and leaves the chiller system at 6°C. Then the delta T is 7°C. A chiller with an operating delta T higher than the design delta T is called a high delta T.
If the delta T of the chiller system is high, it is working properly. The efficiency of the cooling system is also good.
For example, the cooling system has a design delta T of 6°. During chiller operation, water enters the evaporator at 10°C and leaves the chiller system at 7°C. Then the delta T is 3°C. A chiller with an operating delta T lower than the design delta T is called a low delta T.
If the delta T is lower than the manufacturer's specified value, it is low delta T. This is often referred to as low delta T syndrome, as it causes problems with the chiller system.
Air Cooled Screw Chiller
Every chiller system will experience low delta at some point during its continuous operation. Low delta T causes the chilled water pump to run at a faster rate to increase flow. Low delta T reduces the efficiency of the cooling system and consumes more energy.
Low delta T syndrome is the result of under-use of chilled water in AHU/FCU. The causes of low delta T can be
Oversized chillers, pumps, coils, control valves and chilled water piping
Dirty air filters or cooling coils that do not allow air to pass through
Recall that cold water circulates through water pipes throughout the building. The flow of water through the pipes is measured in GPM. the abbreviation GPM is gallons per minute.
GPM indicates how many gallons of water flow through the pipes in one minute. It is highly dependent on the speed and size of the pump.
Refrigeration tonnage is a measure of the cooling capacity of a chiller system. tr is the abbreviation for one ton of refrigeration. In a chiller system, one ton of refrigeration equals 2.4 GPM. to provide one ton of refrigeration, we need a water flow rate of 2.4 gallons per minute.
The maximum height that a pump can circulate is the pump head. The pump head is useful because it tells the pump capacity to do a job. Delta T represents the temperature difference between two points.
The flow of water in a pipe is measured in GPM. In a chiller system, one ton of cooling equals 2.4 GPM. to provide one ton of cooling, we need a water flow rate of 2.4 gallons per minute.