CHILLED BEAMS LESSONS FROM AMERICA
a ‘safety margin’ on top of the estimates, resulting in HVAC systems designed with oversized cooling capacity. In active chilled beam applications, this leads to beams designed to operate with excessive airflows. As a consequence, the active chilled beam often works as an expensive diffuser, with the water valve shut and all cooling provided by primary air. Indeed, beam cooling output is controlled by either a mixing valve, regulating water temperature in the coil, or by an on-off valve modulating water flow through the coil. This valve closes when the space thermostat setting is satisfied. When the system is oversized and primary air provides sufficient space cooling, the water valve stays closed. The authors have observed installations where all of the control valves on active beams were closed throughout the entire summer. Active beam total cooling capacity is the sum
of cooling capacity provided by the primary air and the water flowing through the beam coil. P = Pa
+Pw =mp x cpa (tp (1) Cooling capacity provided by the primary air
is calculated using the following equation: Pa
–tr) (2) Assuming primary air is supplied at 13°C
and space temperature is maintained at 24°C, this provides cooling of about 13 W per L/s primary air. Figure 1 demonstrates the contribution of air (Pa
) and water (Pw ) to the
total cooling capacity of an active beam (P) as a function of primary airflow. As the primary airflow increases, the water contribution to the total beam cooling capacity drops and the air contribution in total beam cooling capacity increases. This chart is representative of a beam designed to operate at fairly low primary airflow. There are chilled beam systems operating at above 30 L/s per metre linear beam with primary air contributing 60% or more to the total beam cooling output. C. Wilkins and M. Hosni1
demonstrated
that plug loads are overestimated for office buildings. This, along with added safety design factor for HVAC equipment, often results in the air-conditioning systems operating at only 80% capacity on a design day. Most active beams are designed as constant air volume systems with water in the coil providing space temperature control. So considering an office space with an active beam sized with primary airflow to cover 60% of total cooling load, assuming 20% safety margin for extra cooling capacity leaves only (100% – 1.2 x 60%) = 28% for cooling output adjustment via cooling coil. This is not enough to respond adequately to the variation in space load in the intermediate season. As a result of this inappropriate sizing, the building will be
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Primary air plenum Primary air nozzles
tp ti2 Coil Supply air qs’ ts tp
qw tw1
tw2 Inducted air qi Figure 1: Cross-sectional view for a typical active beam ti1
Conditioned space tr
overcooled in summer, with thermal comfort compromised and overall HVAC system energy consumption increased. This is not an attribute of the beam but of poor design; when properly applied, beams can practically provide an energy-efficient, low maintenance and comfortable system.
Designing beams for minimum primary airflow The most efficient chilled beam system is the one that operates at minimum primary airflow and satisfies space sensible load primarily by using the cooling coil. The most efficient active beam, by cooling performance, is the one that provides the highest cooling output at minimum primary airflow per unit length of beam. We will define a parameter that represents
this important performance of an active beam and call it Coil output to primary airflow ratio (COPA). COPA represents the amount of cooling (or heating, when active beams are used for heating) produced by the active beam coil per volume of primary air used. COPA is calculated at typical space temperature, inlet water temperature and water flow through the coil.
COPA = Pw qp
The higher the COPA ratio, the more efficient chilled beam design, the more effectively primary air is used. COPA is an important parameter to consider when selecting active beams with primary airflows exceeding minimum outside air requirements. In spaces with high latent load or high outside air requirements, where primary air provides most of the cooling along with dehumidification, application of active beams operating as a constant air volume system becomes less desirable and the COPA ratio loses its importance. As an example, the chart in figure 3 demonstrates the relationship between coil cooling capacity and primary airflow for a
November 2012 CIBSE Journal 47 Supply air qs’ ts
Primary air
qp +
tp ti2 Suspended ceiling
The most efficient chilled beam system is the one that operates at minimum primary airflow and satisfies space sensible load primarily by using the cooling coil
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