Case study: Heat recovery chiller in a new health sciences research lab

In this large laboratory building, a heat recovery chiller is being incorporated to transfer heat from the chilled water being returned from chilled beams into the heating hot water system. The benefit of applying the heat recovery chiller directly to the chilled beam water return — as opposed to applying it to the mixed return water from chilled beams and air handling unit (AHU) coils — is that the chilled beam return water is much warmer, reducing the required lift on the chiller and improving its efficiency.

Chilled water is supplied by the heat recovery chiller to the chilled beams at a temperature that is slightly above the room dewpoint temperature, but below the room’s dry bulb temperature. The room sensible heat that is absorbed into the beam water is transferred by the heat recovery chiller into the building heating hot water system.

Cooling-only chillers provide chilled water to AHU cooling coils at a temperature that is cold enough to dehumidify the incoming outside air when required. The cooling-only chillers can also serve as an additional cooling source for the warmer chilled beam cooling water through a blending valve arrangement.

Outside air is heated, cooled and dehumidified (or humidified, depending on season) at the central AHUs before it is supplied to the laboratories and office spaces at a temperature that is just below  the room design dry bulb temperature. The heating, cooling and dehumidifying process at the central AHUs is done very efficiently, using air to air heat recovery devices to transfer heat to/from the exhaust air (depending on season) and to transfer heat from upstream of the chilled water coil to provide “free” reheat downstream of the chilled water coil, reducing the load on the cooling coil in the process.

The near neutral temperature air is then supplied in an amount required by each room for the greater of the code required ventilation air, volume of dry air required for the latent cooling load, minimum air change rate for laboratory rooms or makeup air for fume hood exhaust. In some laboratories there are large equipment heat gains, which, if a conventional HVAC system had been used, would have required an additional quantity of supply air for cooling and thereby an additional amount of dehumidification and supply fan energy would have been consumed.

Instead, in these rooms the chilled beams remove this additional sensible heat, without adding dehumidification or supply fan energy and the heat is recovered by the heat recovery chiller.

Another benefit of the chilled beam system, as compared to a “conventional” HVAC system in which ventilation air delivery is not decoupled from room cooling is that exhaust air energy recovery can be maximized at the AHU. In a conventional, coupled HVAC system, exterior rooms receive the same cool supply air as interior rooms, so the air being supplied to exterior rooms cannot fully benefit from exhaust air energy recovery devices that are employed at the central AHU to heat incoming outside air.

Further, the cool air supplied by a coupled HVAC system to exterior rooms must be reheated to room temperature before additional heat is added to offset the heat loss through exterior walls. By incorporating a decoupled HVAC distribution system using chilled beams, AHUs can fully use the benefit of exhaust air heat recovery devices to heat incoming ventilation air to near room-neutral temperature before it is supplied to all rooms, including exterior rooms, greatly reducing the need for additional reheat energy.

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