Chilled
beams take advantage of increased volumetric heat capacity of water over air.
Water requires 1/3440th the volume to move the same amount of energy as air at
the same temperature difference. This equates to 1/7th the energy to pump the
water, versus the air, for a load in a standard HVAC system.
An active
chilled beam has two distinct cooling components. The first component is the
induced room air that is cooled by the chilled water coil, and the second is
the primary air. The primary air will be discharged into the chilled beam
through nozzles. As the primary air expands, exiting the nozzles, it will form
a lower pressure zone around the nozzles. This low-pressure zone will induce
air from the room over the chilled beam’s chilled water coil. The induced air
will be cooled by the coil and provide sensible cooling. This is the component
we want to maximize to take advantage of the pumping efficiency / volumetric
heat capacity. If the primary air is supplied at a dry-bulb temperature below
the space temperature, it will provide the second component of sensible
cooling. This component is the same for a standard overhead air system. To summarize,
we want to maximize the induced air. Another way to say that is the higher the
induction rate, the greater the energy savings.
The amount
of induced air is affected by the inlet pressure and nozzle size. A general
rule of thumb is the smaller the nozzle, the greater the induction rate.
The primary
air satisfies three requirements in the chilled beam system. It provides
ventilation air, latent capacity and the energy to operate the chilled beams.
We determined that to maximize energy savings, you want to minimize the
required primary air.
In general,
the primary air’s first requirement, ventilation, will not set the airflow.
Minimum airflow set by ASHRAE 62.1 will not provide adequate latent capacity at
standard commercial supply air temperatures to meet design loads. There are
always exceptions, and design should be looked at on a case-by-case basis.
Latent capacity
of the primary air generally is the driving factor that sets the airflow.
Depressing the specific humidity level of the primary air will increase the
latent capacity of the airflow. This will allow the system to provide less
primary air. The goal is to balance the needs of the space with the minimum
primary air and distribution energy. Ideally, the primary air system should
depress the specific humidity to a level where the required primary airflow is
equal to the ventilation requirements or the chilled beam’s minimum airflow to
meet sensible loads and room coverage.
An
additional advantage to reduced primary airflows is decreased reheat energy. In
maximizing the energy efficiency of the system, the majority of the cooling
load will be controlled by the chilled beam’s chilled water system. As the room
load changes, the chilled beam’s chilled water can be modulated or shutoff to
adjust room cooling. Due to the depressed specific humidity level and the
constant volume supply of the primary air, the latent capacity to the room will
not change, maintaining more consistent occupant comfort even during low
sensible load times without the need for reheat to maintain temperature or humidity.
Reduced
reheat and distribution energy are two ways chilled beams can improve the
energy efficiency of a building design.
Please direct questions toward Titus Communications (communications@titus-hvac.com) and/or Ken Loudermilk, Titus' Senior Chief
Engineer - Sales & Marketing (kloudermilk@titus-hvac.com).
