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We would like to share some information regarding an important change in ventilation requirements for hospital patient areas. These changes present a tremendous energy savings opportunity by using active chilled beams for patient rooms and other areas in hospitals where recirculation of room air is allowed.
The HVAC and lighting energy use in a hospital is more than 2.5 times that in a commercial office building. The single biggest energy consumption (see figure 1 right or click for PDF) is for reheat of supply air which is necessitated by the high ventilation rates in healthcare facilities.
The Department of Energy and ASHRAE have adopted legislation that calls for a 20% energy reduction in existing healthcare facilities and 30% reduction in new construction by 2020. Obviously the reduction of supply air reheat is the prime target for reducing the building energy use.
ANSI/ASHRAE Standard 170-2008 Ventilation of Healthcare Facilities establishes regulations for ventilation rates and practices. This Standard has also been adopted by the American Society of Healthcare Engineers (ASHE). The Standard stipulates ventilation and exhaust rates and strategies for the various areas within these facilities.
In June of this year, ASHRAE and the ASHE approved and published an addendum (addendum h, linked here) to the Standard which revises the ventilation requirements for spaces within healthcare facilities where recirculation of room air is allowable. In general, these areas include diagnostic and treatment rooms, general patient nursing rooms and labor/deliver and post partum areas. Previous to the addendum, these areas required that 6 air changes per hour of conditioned and filtered air be delivered to each space, two of which were outside air.
Addendum h revises the Standard to allow re-circulation of room air to count toward the 6 total air changes, provided:
1) Re-circulation is acceptable in the type of space under consideration.
2) The re-circulation is limited to the room itself and does not include any air form another space.
3) Delivery of a minimum of two air changes (per hour) of outside air (filtered through MERV 13 filter before entering the space) is maintained.
In addition, addendum h stipulates that no filtering of the re-circulation air is required provided it does not pass across a wet coil surface. The addendum is clearly written to promote the use of fan coil units and chilled beams in these areas in order to reduce the amount of supply air reheat.
We have prepared a diagram that illustrates the benefit of active beams in patient areas. Figure 1 (shown above) illustrates an all-air VAV system serving a patient room which is 180 ft2 (1,800 ft3 in volume). The room sensible cooling load at design is 25 BTU/h-ft2. The total ventilation requirement is 6 ACH-1 (180 CFM) of which 2 ACH-1 (60 CFM) must be outside air. The all air system in figure 1 uses an air handling unit to condition, filter and deliver an air mixture that is 2/3 return air and 1/3 outside air. This mixture is delivered 20°F cooler than the room temperature (75°F) to a variable volume terminal with a reheat coil. At full cooling demand, the airflow delivery two the room will be 6.9 ACH-1, about 15% higher than the space minimum ventilation rate of 6 ACH-1. Such a high sensible cooling demand will rarely be required and in fact the average cooling demand may only be 30 to 40% of this design value. The VAV terminal will throttle the supply airflow during periods of reduced demand, but can only reduce the airflow to 6 ACH-1 due to the space ventilation requirements. Any time the space cooling demand is less than that delivered by the mandated 6 ACH-1, the supply air will have to be reheated prior to its delivery to the space to meet the instantaneous space cooling demand. This will be necessary any time the space cooling demand is less than 85% of its design (maximum) value.
Figure 2 (shown left and in diagram) illustrates the same space being supplied by an active chilled beam system. Note that only the 2 ACH-1 of outside air is ducted from the AHU, thus it becomes a dedicated outdoor air system (DOAS). This air is conditioned and filtered then delivered at 55°F to a 6 foot long DID632 beam with "M" nozzles. Such a beam is capable of inducing 4.4 CFM of room air across its heat transfer coil for every CFM of ducted air it is supplied.
When the mandated 2 ACH-1 of outside air is delivered to the beam, a discharge volume flow rate of 10.8 ACH-1 is maintained to the space. At design conditions (25 BTU/h-ft2), the ducted (primary) air will provide only about 28% of the sensible cooling while the remaining 72% will be provided by the reconditioning of room air within the beam's chilled water coil. During periods of reduced cooling demand, the coil chilled water flow rate will be throttled while the ducted and total discharge airflow delivery remains constant. Not until the coil contribution is reduced to zero will reheat of the ducted air supply be required. In this case, reheat will not be required until the space cooling demand has been reduced to about 29% of its design value.
This is a tremendous energy savings opportunity for engineers, architects and owners who are involved in the design and operation of health care facilities. It is also a tremendous boost for you to sell more chilled beams. TROX USA had a great deal of influence in bringing this addendum to its approved status. Let's get out and take advantage of this effort by making your clients aware of these monumental changes. As always, you can count on our support in designing these energy efficient solutions.
If you have any questions about this technical brief, please contact us at 770.569.1433.
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