Chillers, AHU, RTU How they work

Chillers, AHU, RTU How they work

Chiller and AHU and RTU how they work

In this article we will be looking at how chillers, AHU’s and RTU’s work together, the difference between them and the basic working principles behind each of them. Chillers, Air Handling Units and Rooftop units are used extensively within HVAC systems for commercial properties. 


Water and Air cooled chillers

Chillers are used to generate chilled, or cold, water which is pumped around the building to provide air conditioning by collecting unwanted heat.

There are two main types of chillers, Air cooled and Water cooled. The term ( AIR ) or ( WATER ) cooled just tells the engineer how the chillers condenser is rejecting heat from the building.

Water cooled Chillers

Water cooled chillers are typically located in the basement or lowest floor of a building. This type of chiller needs a cooling tower to reject the heat from the building. The chiller produces chilled water and pushes this around the building to Air Handling Units (AHU’s) and Fan Coil Units (FCU’s) etc. These units circulate air around the local space as well as the building. The air is forced across heat exchangers, containing the chilled water, which extracts the unwanted heat before the air is distributed throughout the building.

The unwanted heat, that is extracted from the air, collects in the chilled water loop. This loop circulates back to the chiller and once it reaches the evaporator, the unwanted heat will  be transferred over to the chillers condenser via a refrigerant loop.

The condenser absorbs this heat and then dumps it into the “condenser water” loop which runs between the chillers condenser and the cooling tower. The cooling tower will force ambient air across the condenser water to extract the unwanted heat. The fan within the cooling tower forces this heat to leave the system, and building, and is ejected into the atmosphere. In this instance the condenser of the chiller has been cooled by water so it is a water cooled chiller.

Water cooled chillers are typically used on large commercial properties with high cooling loads, you can tell if a building has a water cooled chiller because it will need cooling towers which are typically on the roof. There will usually be more than 1 chiller and cooling tower, its not uncommon to have at least 2 or 3 or even more just for redundancy as well as variations in cooling demand.

Air cooled chillers

Air cooled chillers are typically located on the roof of the building or outside e.g. in the car park. Air cooled chillers also generate chilled water which is pumped around the building to air handling units as well as other apparatus such as fan coil units etc. This is to cool the building down by picking up unwanted heat.

The air is forced to circulate around the building, and local space, and will pass across heat exchangers within the AHU and FCU’s. This unwanted heat is again transferred over to the condenser of the chiller from the evaporator. It is transferred via a refrigerant which loops continuously between the evaporator and condenser and is forced by the compressor. The difference with this type of chiller is that a fan blows air over the condenser which removes the heat. So in this instance the condenser of the chiller has been cooled by air, therefore it is an air cooled chiller.

Air Handing units and Rooftop units

These two system both serve a very similar purpose and can look fairly alike. Both of these are used to distribute air around the building and will contain fans to achieve this. They will both take in fresh ambient air and clean this through a filter before heating or cooling the air to suit the demands within the building. The main difference between the two is how the air is heated or cooled and we’ll look at each of these to learn the basics of how the work. 

Air handing units ( AHU'S )

Air handling units are also referred to as AHU’s for short. AHU’s are typically located inside the building although you can also find these within rooftop plant rooms and its becoming more common to find robust, weather tight, AHU’s sitting out in the open on the roof, to free up valuable space inside the building. Their purpose is to distribute air around defined areas within the building. 

Sometimes one AHU will supply the whole building but its common in newer buildings to have multiple smaller AHU’s feeding different parts to provide a better indoor environment as well as energy savings. AHU’s typically do not have their own cooling system built in, instead they will be connected to a central plant water or air cooled chiller, occasionally they will use some sort of split a/c unit and you can also find these connected to district cooling networks.

How AHU Works 

AHU’s are connected to duct work which provides a defined route for the air to travel around the building. In a simple form, an AHU will use a fan to suck in fresh ambient outside air which will then pass through some filters to remove any dirt and dust. It will then pass through the cooling and heating coil heat exchangers which, as discussed, are typically fed from central plant units. Once the air has passed over these coils it will be pushed out around the building. Another set of duct work will be collecting the warm used air from the rooms and will bring this back to the AHU via another fan. This fan will then eject the air from the AHU and building.

Roof Top Units ( RTU'S ) 

These units are always located on the roof, as the name suggests. They are very common in shops and small commercial properties and are popular because they are simple, compact, self contained, all in one HVAC units. Their purpose is to distribute conditioned air around define areas within a building. RTU’s are also connected to duct work which provides a defined route for the conditioned air to travel along. RTU’s typically have their own inbuilt refrigeration system to provide cooling only, but they can also contain a heat pump or a separate heat source such as gas fired heater. 

RTU’s house all the components within the rooftop unit. This contains a number of dampers to control the flow of air and can allow air re circulation, if conditions are right. The unit will also contain filters, to clean the air, as well as heating and cooling coils and at least 1 central fan. The refrigeration system is also connected, typically to the back or side, to control the temperature. 

The fresh ambient air is sucked in via the fan and passes across the dampers. It then passes through the filters to remove any dust and dirt before its temperature is adjusted in the heat exchanger. The fan then pushes this out into the building to condition the space. 
Another duct will collect the warm used air from the rooms and will distribute this back to the rooftop unit where it will be ejected or recirculated, if that option is available and conditions are right.

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