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When designing an HVAC system there are a number of different choices that have to be made. One such choice is the type of terminal unit to use meet set requirements. There are many factors that influence the choice, such as energy consumption, service need, ease of system design and installation. Below are pros and cons of active chilled beams and fan coils, two competing types of terminal units.
Active chilled beams and fan coils are two different types of terminal units that are used to control the temperature in a room. Both are equipped with a coil that exchanges energy between the room air and the water flowing through the coil. The latter can either be warmer than the room air, to deliver heat to the room, or it can be colder and thus remove heat from the room.
Fan coil units (FCU) can be connected to a ventilation duct and be used to distribute primary air in a room, however it is not necessary for controlling the room temperature. In an FCU, air from room is forced to circulate through the coil using a fan. Whereas an active chilled beam (ACB) needs to be connected to a ventilation system and has to rely on the fan of the air handling unit to build up pressure in the duct system. The primary air is distributed through small openings in the pressure chamber of the active chilled beam and further into the mixing chamber that creates a zone of low pressure which draws room air through the coil. This is referred to as induction and works without any moving parts in the ACB.
How wet and dry cooling can impact the choice
Active chilled beams are often considered to be an energy-efficient way to provide heating and cooling to a room. The main reason for this is that they are designed to operate in a dry cooling mode, meaning that the supply water temperature while cooling is always kept above dew point in the room. This makes it possible to operate a chiller at a very high efficiency. Traditional installations of chilled beams have relied on a constant flow of supply air to ensure good indoor air quality. The latest generation of ACBs have been developed to operate with functionalities such as variable air volume (VAV) and demand controlled ventilation (DCV) to order to increase energy-efficiency further.
FCUs on the other hand are often designed for wet cooling, meaning they operate with much lower supply water temperatures. Thanks to the lower water temperature and the built-in fan that circulates a lot of room air, FCUs can provide a high cooling capacity from a compact unit but, at a greater energy cost compared to ACBs. The increased energy consumption is in part due to reduced efficiency of the chiller plant to produce colder water, and in part to the energy required to condense moisture in the circulate room air. It is possible to operate FCUs under dry cooling conditions. However, the fan must then run at a higher speed which can result in problems with noise and air velocities in the occupied space, which reduces the quality of the indoor climate. Modern fans are relatively energy-efficient, meaning the added energy consumption is quite moderate, but with FCUs in every room the energy consumption will add up. Further, fans are not perfectly efficient, some fan energy is turned into heat which needs to be removed during the cooling season. Recently developed and modern fans are fairly quiet when they are new, and especially when they operate at lower fan speeds. However, fans that run at full speed, and especially fans that are approaching end of their life, generate noise which is undesirable in all building applications, but especially in hotel rooms and offices.
Installation and maintenance, a cost today or tomorrow
Normally, an active chilled beam has no moving parts and thus poses very little need for service. It is recommended to vacuum the coil to remove dust that otherwise can build up over time on the surface of the coil. This needs to be done according to building conditions. Some buildings will require quarterly cleaning, other will only need cleaning every other year. ACBs have a very long lifespan and very little need for service, making them a cost-effective solution considering the products’ lifetime. In a more advanced active chilled beam it is possible to adjust the amount of supply air provided to a room, which means that it can be adapted according to a changed use of the space. That in turn reduces the need to replace units, parts and components when a building is being refurbished, consequently embodied carbon can be limited over the building life cycle.
An FCU that operates under wet cooling conditions is equipped with filters to prevent dust getting stuck on the wet coil. It is also equipped with drain pans and a drainage system to get rid of the water condensed out of the recirculated room air out of the room air that is circulated through the coil. Besides the fan, both filter and the drainage system are service points that will require maintenance. A drainage system that is not taken care of can lead to growth of mould and bacteria which in turn can lead to unpleasant smells and health hazards.
An ACB needs to be connected to the ventilation duct, whereas this is optional for an FCU installation, this because ventilation can optionally be distributed through separate diffusers instead. The installation of piping is relatively similar for active chilled beams and fan coil units, it can be either two- or four-pipes connected to the coil in the terminal unit. The main difference is then that an FCU that is intended to operate under wet conditions needs to have insulated pipes on the cooling side for both supply and return water. Other than that, an active chilled beam installation will only require the fitting of a control system to modulate the flow of water through the coil and possibly VAV supply air. An FCU will also require connection to a control system to manage water valves and fan-speed. However, a fan-coil unit will in addition require power for the fan and a drainage system in place which increase the installation cost.
The advantages in sum
In cases where ventilation is not required, active chilled beams turn out to be a very expensive solution. Another advantage of the FCU is that it is relatively easy to construct a system and select terminal units. To design an active chilled beam solution require more attention to handle humidity and control supply water temperatures. The high heating and cooling capacity of a FCU is often seen as an advantage and a possibility to save energy, as the room temperature can be allowed to drift further away from the desired set-point during periods of no occupancy. This might save energy, especially in buildings with less thermal inertia, but it then comes at the cost of higher power consumption, noise and risk of draught.
It is clear that both active chilled beams and fan-coil units have positive and negative characteristics and there will always be cases where one or the other will be the preferred choice of terminal unit. In a situation where there is a high demand on the requirements of the indoor environmental quality, active chilled beams are often a preferred solution that offers excellent thermal comfort, good indoor air quality, low sound levels and minimal energy consumption. An FCU usually have a lower initial cost per W (or BTU/H) of cooling, however, considering reduced operating and maintenance cost combined with a long life expectancy, typically results in a favorable life cycle cost for ACBs.
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