Fan coils – FCU

 

Constant air volume, CAV. In this article, we are going to look at the CAV system. CAV stands for constant air volume. CAV is a means of providing air conditioning around a building. This is becoming less common in new buildings simply because VAV, Variable Air Volume, systems are replacing them due to their superior zone control and greatly reduced energy consumption.

Scroll down for the YouTube tutorial on CAV systems.

Although constant air volume systems are becoming less common in new buildings, you can still find them in smaller buildings; especially the older ones. The reason you will find them in smaller buildings is that they are fairly inexpensive to install and are much easier to install and use. Additionally, hospitals will often continue to use these systems for their operating rooms.

Looking at a CAV system, we have a simple model of a small office below.

We have the AHU, Air Handling Unit, which is located in the technical room on the left side. Then we have the supply duct, which takes all the fresh air treated from the CTA and transports it throughout the building. The treated supply air leaves the duct in the rooms through the diffusers. It is circulated through the room and used dirty air is drawn into the return duct through a grill located in another part of the room. This air will be sent back to the CTA where it can be dispersed directly into the atmosphere or some part can be recycled and reused.

Now, in this video we are not going to look inside the AHU to see the components as we have already covered in other videos. If this interests you, I recommend you check out our “Fundamentals of HVAC” where we cover this in detail.

CAV systems have certain limitations because the supply air temperature varies, but the volumetric flow rate (delivery) of the air is constant.

While the system is running, it delivers air at a constant, constant volume and only the temperature of that air changes. Air is typically supplied at around 13 degrees Celsius (55 degrees Fahrenheit), but this can be adjusted based on the needs of the building. Now, like I said, the volume is constant, but how much air you deliver will really depend on the size of the area being cooled. The activity that occurs in rooms will dictate the amount of air that needs to be supplied to a room.

However, to give you an idea, the small room in the middle of the illustration above might require something like 3 cubic meters per second. However, the larger rooms on either side may require around 20-30 cubic meters per second. It could be that the entire building only requires 30 cubic meters per second. But it can be much more than that. Suppose we have a 40-story building with a single huge AHU on the roof, then it will have to deliver an extremely large volume of air per second.

One of the problems with the CAV system is that everything connected to this AHU is classified as a single zone, which means that all the rooms connected to it receive the same air temperature regardless of their thermal load. So if a room were a busy meeting room in the height of summer and full of people, it would generate a lot of heat. It also has solar thermal gain. But with a CAV system, this room will receive the same air temperature as the next empty room. This means that zones receive a certain amount of cooling, regardless of their needs. This is not efficient as you will waste a lot of cooling energy, generating and providing cooling for no reason.

The fans would also run at 100% for the entire system run time. Though you can install some VSD/VFD/Frequency Inverters, to reduce motor speed.

It is very difficult, but not impossible, to convert a CAV system to a VAV system. It will take a lot of time and effort. You have to change a lot of things and modify a lot of controls for that. The conversion can unlock many energy savings, although a CAV is cheaper to install and easier to operate.

From this schematic representation of the CAV system, you can see how it is connected. Please note that all rooms are connected to the main duct and the only form of climate control is at the main CTA. This means that all rooms receive the same warm air at a constant volume.

The CAV design will work well if all parts are in similar conditions or require similar cooling/heating conditions. But if a room has a different cooling/heating load, it may need to be connected to a different CTA. Depending on the size, it may be better to have your own AHU power supply.

One solution is to install terminal heaters. These are usually found in a metal box on the ceiling, before the diffuser. They are usually electric heaters. They can also come from the hot water system. These will heat the air to a higher temperature, but this is obviously very energy inefficient as you are already cooling the air in the AHU and now you are paying again to heat that air. So you're wasting money on cooling as well as heating.

Typically, the air temperature for this type of system is supplied at the coldest possible temperature suitable for the room with the highest cooling load. Looking at the illustration, the largest room will likely have the highest cooling load, but the other rooms will have the heater installed.

Another option you might see in the industry is the dual duct CAV system.

It's getting rarer to see this, but you can still find it in some older buildings. But basically you have two ducts running and supplying air to the room, and one of them will supply cool air and the other duct will supply hot air. This air is then mixed by dampers in a box for each room just to match the inside temperature. This return air is also returned to the CTA, ready to be recycled or dispersed into the atmosphere.

This system provides better thermal control but there is little humidity control. Again, it's not particularly energy efficient either. Since you are supplying air through two streams, you have a lot of resistance that the fan will have to overcome. You're also needlessly heating and cooling air that might be mixed.

To make the dual duct system more energy efficient, you must ensure that a temperature reset is enabled on the controls. You will need some advanced checkpoints and some temperature measurement sensors. The temperature reset controls will monitor the required air conditions and then reduce the temperature of the hot air to bring that temperature down to the lowest acceptable temperature. The controls will also increase the temperature of the cold air to reduce the cooling load provided by the coil.

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