Dedicated outdoor air unit Primary air to the beams Return air
Parallel beam booster terminal
In active beams with induced air moving horizontally across the coil, coefficient b in Equation 7 becomes zero because velocity across the coil is not affected by buoyancy force and Equation 8 is reduced to: K’=a(Kin
x qp x ƿi ωn2 ) n1 (8a) Equation 8a can also be used to define the
heat transfer coefficient for passive beams, where airflow through the coil is determined by convection forces only as represented by the following equation. K’ =a(∆tn
x ƿi ωn2 ) n1 (8b) Equation 8 and its derivatives are important Active beam Figure 4: Variable air volume beams with the parallel boosting terminal
for understanding what parameters affect coil cooling or heating output. They provide sufficient information to simulate any active beam in energy simulation software. All
Dedicated outdoor air unit Inline beam booster terminal Return air
empirical coefficients a, c, n, n1, n2 and Kin are constant for a given beam design and can be derived from the manufacturer’s cooling and heating (when testing active beams for heating) capacity tests. The test sequence, along with the calculation procedure to determine these coefficients, can be part of the method of tests for active beams currently being developed by ASHRAE. This would help integration of active beam systems in the energy simulation software. Power factor n1 in these equations is three
to four times higher than power factor n2. That leads to the conclusion that increasing Kin
has
a major effect on the COPA. The higher induced airflow through the coil, the higher the coil output, hence the higher the COPA.
Active beam Figure 5: Variable air volume beams with the inline boosting terminal
account active beam induction force and the second pair consider the buoyancy force acting on non-isothermal air moving in a vertical direction across the coil. For example, if warm induced air moves up across the coil, it cools down and buoyancy force slows its motion. On the contrary, if active beam design deploys downward movement of induced air, this buoyancy force will be accelerating the air movement across the coil when in cooling mode.
v = Kin x qp + b x ∆t Af (7)
Combining Equations 6a and 7, and taking into consideration that Af
is constant for a
given beam design, an equation is defined relating coil heat transfer coefficient as function of temperature difference ∆t, velocity of water in the pipes ω and primary airflow qp + Kin x qp)ƿi ωn2
. K’=a [(c ∆ tn 50 CIBSE Journal November 2012 ] n1 (8)
The evolution of active beams: VAV beams A fan-assisted, or VAV, beam uses a built-in fan to increase the circulation of room air during peak loads. Such a system designed for hotel applications is used to boost the cooling or heating output of a beam by up to 30% during peak load hours, and to accelerate room conditioning at the start of ‘occupied’ mode. Figures 4 and 5 show beam booster terminals
(BBT) that increase the cooling/heating output for a group of beams. A BBT, fitted with a cooling coil and a condensate drain, can also be used for dehumidification in case of excessive latent load in a zone served by this unit. The parallel booster terminal (Figure 4)
is similar in design to a fan-coil unit with a variable speed fan, increasing return airflow and boosting cooling/heating output. It is similar to a fan-powered terminal, but induced airflow can be controlled independently of primary airflow. As shown in Figure 5, the inline BBT requires no additional ductwork and can work with regular active beams. It relies primarily on increased
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