The article "The Quest for Maximum Boiler-System Efficiency" by Senior Editor Ron Rajecki (June 2011, http://bit.ly/Rajecki_0611) is very well-written. When I tried to push the idea of using delta-T in lieu of delta-P to control variable-speed pumps in a school hydronic system, engineers said they were afraid some remote coil may not get enough water pressure to satisfy coil demand.
Been Kuo, PE
Ohio School Facilities Commission
Response from one of the article's sources:
In larger commercial buildings, some terminal units may not get enough flow with delta-T control of pumps. This could happen in a two-pipe system that was not balanced properly.
In a two-pipe reverse-return system, this is unlikely. If the system was designed with a single-pipe system or you are using a two-pipe direct-return system with automatic balance valves, this will not happen because every one gets the required amount of water at all times.
Greg Cunniff, PE
The April 2011 article "Energy-Recovery System Reduces Cost of Cooling Outside Air by 70 Percent" (Design Solutions, http://bit.ly/Design_0411), about Turtle River Montessori in Jupiter, Fla., says 7,500 cfm of outdoor air is supplied for 175 occupants. That is 43 cfm per person. ANSI/ASHRAE Standard 62.1-2007, Ventilation for Acceptable Indoor Air Quality, says outdoor-air quantity should be 13 to 17 cfm per person, depending on how a building is classified. The article does not say why or how 43 cfm per person of outdoor air is required for this building.
A recent article in HPAC Engineering—"Making a Case for Reduced Classroom Ventilation" (Managing Your Facilities, December 2010, http://bit.ly/Managing_1210)—shows that 5 to 7.5 cfm per person is quite adequate in Florida schools.
Data in the (April) article show the "recovery-efficiency ratio" of the heat wheel to be 90. Yet reference to the AHRI (Air Conditioning, Heating, and Refrigeration Institute) directory for this manufacturer shows the highest recoveries are in the 70s.
The article claims estimated ongoing savings are $12,000 per year; the savings attributed to the heat wheel alone amount to 57 cents per square foot per year. The article does not say how those savings were estimated or determined. Experience tells me total electricity cost in Florida schools is $1.20 to $1.40 per square foot per year, and the electricity cost for new code-compliant schools should be much lower. Because these electricity costs include lights and equipment, it is necessary to look for documented end-use electricity data. The best source of this data is the U.S. Energy Information Administration’s Commercial Buildings Energy Consumption Survey (CBECS).
The latest CBECS shows the average total energy cost for education buildings of all ages and types is $1.22 per square foot (Table C4); in the south, the cost is $1.24 per square foot (Table C6). End-use data show education buildings use 8,000 Btu (Table E2A) and 2.2 kwh (Table E6A) per square foot per year for cooling. Table E6A shows total electricity use for education buildings of all types and ages to be 11.0 kwh per square foot per year.
Table E6A of the CBECS shows the following kilowatt-hours per square foot per year: ventilation, 2.5; water heating, 0.3; lighting, 3.4; refrigeration, 0.5; office equipment, 0.1; computers, 1.0; and other, 0.6, for a total of 8.4 kwh per square foot per year. At 10 cents per kilowatt-hour, that leaves 2.6 kwh per square foot per year, or 26 cents per square foot per year, for cooling. Assuming all existing Florida schools use even twice as much as the average for cooling, the total cooling cost still is only 52 cents per square foot per year. Certainly, the cooling cost in new code-compliant schools must be less than that. How can this article claim cooling-energy-cost savings of more than most schools use?
The cooling savings claimed in the article should result in the utility company sending checks to the school every month. The school has been in operation for over a year. Including data from actual utility bills would have provided a basis for comparison with other schools in the area.
Larry Spielvogel, PE, FASHRAE
Bala Cynwyd, Pa.
The information provided in the case study was collected from KAMM Engineers, JCI/York representatives, MicroMetl, and the building's owner, Ms. Bubli Dandiya. To the best of our knowledge, this information accurately reflects the performance of the HVAC system and the Airxchange energy-recovery wheel installed at Turtle River Montessori.
Runaround-Coil Heat Recovery
I read with great interest the January 2011 article "Evaporative-Cooling-Enhanced Runaround-Coil Heat Recovery" (http://bit.ly/Shah_0111). I have been involved in similar designs for several decades.
We pump cooling-coil condensate over PVC (polyvinyl chloride) fill located in the exhaust-air stream before the heat-recovery coil, where the cold condensate evaporates, depressing exhaust-air dry-bulb temperature. We estimate the reuse of condensate improves overall heat-recovery efficiency from 55 percent to about 74 percent. This scheme transforms a normally sensible-only heat-recovery system to a total heat-recovery system with total separation of air streams.
Cooling condensate is a perfect source of water because it is free, devoid of deposits that may foul the evaporative pad, and cold (about 60°F). Also, the cooling coil produces the maximum condensate flow when needed most at high outside ambient wet-bulb temperatures. Because the facilities usually have a steam boiler for serialization and humidification, de-ionized boiler makeup water provides calcium-free water to the evaporative fill when cooling-coil condensate flow is insufficient. Even in Pittsburgh's semi-humid summer climate, the evaporative-cooling effect of the heat-recovery system is of great benefit.
We have used this concept on several laboratory projects requiring total separation of hazardous exhaust from makeup air. We "stack" the exhaust-air heat-recovery unit on top of the makeup-air unit, producing a total heat-recovery system that saves space and provides good exhaust-air discharge velocity, preventing recirculation of laboratory exhaust into the outside-air intakes. Around the heat-recovery coils, we use a bypass damper that opens when no heat recovery is necessary, reducing "parasitic" coil air-pressure losses when the coils are not needed.
We first applied this system in 1988 for the University of Pittsburgh's Rangos Research Center, a 110,000-sq-ft biotechnology research laboratory retrofitted into a 10-story warehouse. In 1991, we installed the system for the Magee-Womens Research Institute, a 110,000-sq-ft medical research laboratory in Pittsburgh. Most recently, we did the conceptual design for upgrades to the University of Pittsburgh’s Chevron Science Center. This is a 300,000-cfm, 100-percent-outside-air system with stacked units on the roof serving graduate research laboratories.
This is a great concept that should be considered for any 100-percent-makeup-air system.
Mark S. Wolfgang, PE, LEED AP BD+C
Loftus Engineers LLC
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