ASHRAE Journal - July 2012 - 10

LETTERS
Heat Pumps for Centralized Plants
I found May’s “Large-Capacity, Water-to-Water Heat Pumps for Centralized Plants” by Craig Campbell, Joe Catrambone and Chris Paraskevakos to be very useful. It provides a good basis for sizing a water source heat pump, and it does a good job of explaining the need to operate a water source heat pump in heating mode in warm weather and in cooling mode in cold weather. However, it does not address integration of the heat pump with operation of the chiller to prevent operating the chiller at very low load, which is inefﬁcient and can cause surge problems. Figure 2 in the article shows an example where there is signiﬁcant cooling load at all outside air temperatures, but when the heat pump is operated as proposed, the supplemental cooling load left for the chiller drops to zero. Clearly, the chiller will need to be turned off well before that happens. To avoid a shortfall of chilling capacity, it would be preferable to integrate the controls of the heat pump and chiller to keep both units operating within reasonable load ranges until the heat pump can provide adequate cooling to allow the chiller to be shut off. Jerry Smith, P.E., Member ASHRAE, Fulshear, Texas
THE MAGAZINE OF HVAC&R TECHNOLOGY AND APPLICATIONS ASHRAE.ORG
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MAY 2012

JOURNAL
THE MAGAZINE OF HVAC&R TECHNOLOGY AND APPLICATIONS APRIL 2012

Economics of AHU

Designing for IAQ in

Don’t Turn Active Beams Into Expensive Diffusers | Slab Happy |

Large-Capacity, Water-to-Water Heat Pumps | HVAC for Schools | Backward-Curved Fans | New Legionella Standard | Hot, Humid Building Assemblies

Air Outlet Performance | Shielding Fresh Air Ventilation Intakes

the operation within the operating envelope of the heat pump. Because this switchover could occur multiple times during a day, it is advantageous if the heat pump can handle the operating mode switch automatically. Craig Campbell, Joe Catrambone, Chris Paraskevakos, Member ASHRAE, Glendale, Wis.

Designing IAQ in Natatoriums
I read April’s “Designing for IAQ in Natatoriums” by Randy Baxter with interest. I was disturbed by the explanation of why exhausting from the pool water surface works. The speciﬁc gravity of the pollutant gas, trichloramine, will have a very small effect on the density of the layer of air adjacent to the water surface. Consider that a cubic meter of dry air weighs 1.2 kg at standard conditions. The highest measured concentration of trichloramine mentioned in the article is 18.8 mg per cubic meter. Even if that trichloramine did not displace any air, it would increase the weight of the cubic meter of air to 1.2000188 kg. That difference in density will have no impact on where the contaminated air goes. Exhausting from the surface of the water works well because that is where the trichloramine is coming from. That will also be where the concentration is highest, not because the trichloramine is heavier than air, but because that is where it is released to the air. I look forward to more articles documenting the beneﬁts of exhausting from the surface of swimming pools. Ed Chessor, P.E., Life Member ASHRAE, Vancouver, BC, Canada

The Authors Respond
Mr. Smith’s observation is correct: at the load point where the chiller would cycle on because the heat pump is not capable of handling the entire cooling load, there exists the possibility of the chiller being asked to run below its minimum capacity. In this case, the leaving chilled-water temperature (LCHWT) setpoint of the heat pump can be increased (or the ﬂow reduced) temporarily, reducing the cooling load on the heat pump, and making more load available for the chiller. Of course, the heat pump will also provide less heat during this period. If this reduction in heating capacity is unacceptable, and the chiller is equipped with hot-gas bypass (HGBP), it could operate down to as low as zero effective load through this transient period. While falsely loading a chiller with HGBP runs contrary to the energy-saving purpose of installing the heat pump in the ﬁrst place, we must remember two things: a) it is still more efﬁcient than operating a boiler, which is the least efﬁcient component in the system, and b) the situation is only temporary, until the cooling requirement is sufﬁcient. This may sound complicated, but control of simultaneous heating-and-cooling installations is always more complex than conventional HVAC systems. The equipment controls for the heat pump must be capable of switching between heating priority and cooling priority modes, while controlling the load(s) in response to the system requirements. Regardless of the system arrangement, there will always be design setpoints for the leaving hot water temperature and the LCHWT, even if the chilled water is not produced for beneﬁcial use. The control system must honor both of these setpoint limits, so as to conﬁne
10 ASHRAE Journal

The Author Responds
Mr. Chessor is correct that the density of fully diffused and diluted trichloramine-containing air is not signiﬁcantly different from that of trichloramine-free air. But I disagree that the difference in the density of air and trichloramine has no impact on where the contaminated air goes. We agree that the source of trichloramine is the water in the pool and that the concentration will be higher at the water
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ASHRAE Journal - July 2012

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