Air-conditioning in a central air-conditioning plant
In this module, you will discover the three most commonly used air-conditioning systems in the air-conditioning industry and get a thorough insight into the various components of a central air-conditioning plant.
Air-conditioning is a critical part of any commercial building. In this module, we're going to take you through the fundamentals of air-conditioning, covering the most common types of air-conditioning systems, as well as taking a closer look at the components and pumps used in a central air-conditioning plant.
There are three main types of air-conditioning systems.
First of all, there are individual or room air-conditioners. These are used for residential applications serving one or two rooms with limited capacity up to approximately 5 tonnes or 17 kW of cooling.
Secondly, there are variable refrigerant flowsystems or VRFs. These are used in light commercial installations in places such as restaurants or small offices and they consist of fan coil units, refrigerant piping and ductwork.
Finally, there are the central chilled water air-conditioning systems. These are used in large commercial or industrial establishments that consist of major components such as chillers, pumps, air handling units, cooling towers, control valves and pipe networks.
Now, let's turn our attention to the various subsystems in a typical chilled water air-conditioning plant.
A chilled water air-conditioning plant typically consists of 5 main circuits: Refrigeration, heat rejection circuit, chilled water circuit, terminal units for airdistribution and controls.
We're going to take a look at them one by one.
First up, there's the refrigeration circuit. This circuit includes a chiller, consisting of a condensor and evaporator, in which the refrigerant changes its state at various stages to either absorb or reject heat from the evaporator and condenser, respectively.
Next, there's the heat rejection circuit, which consists of cooling
towers, condenser water pumps and water treatment systems.
Then, there's the chilled water circuit. It consists of chilled water distribution pumps which circulates chilled water to terminal units such as Air Handling Units (AHUs), Fan Coil Units (FCUs) or Ceiling Suspended Units (CSUs).
The airhandlers, fancoils and ceiling suspended units transfer heat from the air to the chilled water either directly or through a network of airducts.
Finally, there's the control circuit. The control circuit integrates all the sub-systems of an HVAC plant into a single system for easy monitoring and control. This includes chillers, pumps, cooling towers, terminal units, cooling tower fans and water treatment systems. If the system isn't connected, it works as a stand-alone system, which can be very inefficient. A connected system is both more efficient and more reliable - increasing your peace of mind.
Now we know what a central air-conditioning plant consists of, it's time to take a look at what role pumps play in the plant.
Despite being responsible for just 5% of a plant's entire energy consumption, pumps are certainly the heart of every chilled water plant. Pumps influence how all the equipment in the building performs. In other words, the performance of the chillers, cooling towers, air handling units and fan coil units is directly related to the performance of the pump.
In a central airconditioning plant, pumps can be used in both a chilled water distribution circuit which is a closed system and in the evaporative condenser water circulation circuit system which is an open system.
When installing chilled water pumps in a closed loop hydraulic circuit, the water in the pipeline is prefilled by an external make-up water system.
Here, the pumps are sized to overcome the frictional resistance offered by the pipe network, valves, bends and cooling coils in the chillers and air-handling units. On the other hand, installing condensor water pumps in an open loop circuit exposes the pipe network to atmospheric pressure at the cooling tower.
Here, the pumps are also sized to overcome the frictional resistance offered by the pipe network, valves and bends. But unlike in a closed loop circuit, pumps in an open loop circuit should overcome the static height of the cooling tower as well.
Finally, in any open loop circuits, the NPSHa - the Net Positive Suction Head available from the system - should always be higher than the NPSH required by the pump. That way, you can avoid damaging cavitation.
That covers our module about some of the fundamentals of air-conditioning systems.