Enhanced Laboratory HVAC Systems
October 1, 2007
In addition to promoting the efficiency and economy of providing a fancoil unit for each lab space to complement a more typical VAV approach, the author also submits a few tips for the design and submittal process for consultants.
Since the early 1980s, the attention to more energy efficient laboratory designs has been on the rise due to the increase in energy cost and the emergence of the greening umbrella that has cast its shadow on different HVAC applications. Since VAV systems entered the laboratory HVAC design in the early 1980’s, professional organizations such as ASHRAE, Association of Energy Engineers, LEED®, R&D Magazine, and Lab 21 have started establishing professional development programs in order to bring innovative designs in the field of laboratory VAV systems.
Up to this time, technology had not yet made a drastic change in the laboratory VAV design. The typical laboratory VAV system still has 100% outside air. Installation costs are still high compared to office or commercial building HVAC systems. In addition, energy consumption remains on the rise.
The purpose of this article is to introduce a new concept in the VAV design of laboratory HVAC systems. This concept provides a separate, independent makeup air unit for each floor in lieu of one large centralized makeup air unit for the entire building (Figure 1). The purpose of the independent AHU is to provide makeup air for the exhausted air from fumehoods on that floor and to positively pressurize the corridor with a constant flow VAV box with a reheat coil.
In addition, each laboratory space is provided with its own VAV fancoil unit (FCU).
The operation of this VAV/FCU is enhanced through the energy savings accomplished by utilizing this system (Figures 2 and 3). The concept is not actually new since it has been partially in use at several laboratories in the natural science building at a major U.S. university.
Building A CaseIn order to compare the advantages and disadvantages between the conventional lab VAV systems vs. the VAV/FCU system with an independent makeup air unit for each floor, a four-story laboratory building with 28 fume hoods is selected. For simplicity, each hood exhausts 700 cfm at full open sash and 250 cfm at closed sash. Face velocity is 100 fpm at any sash position. Each FCU supply air is 600 cfm maximum and 150 cfm minimum in capacity. There are seven FCUs on each floor. Also, each floor is provided with fixed 900 cfm for corridor pressurization, a reheat coil, and a thermostat. The capacity for each floor’s makeup AHU is 5,100 cfm maximum and 1,950 cfm minimum. The lab space module is 24-ft long by 12-ft wide by 8.5-ft high, and there is a total of 28 equal lab modules.
How often is the fume hood sash raised to full open position? Per the laboratory user’s survey conducted by BD&C/RICS in September 2003 (Figure 4), the actual hood full sash opening per 8 hrs/day is estimated to be 32.4%, which is equivalent to 2.6 hrs out of 8 hrs/day. Therefore, 5.4 hrs out of 8 hrs/day and the rest of the 16 hrs of the day, the hood sash is expected to be in the closed position.
The exhaust cfm through the hood is a minimum of 250 cfm, with 450 cfm is returned air (RA) from the lab space to the FCU to mix with the 150 cfm from the makeup air unit. Estimated cooling thermal energy saved, in Btuh, is as follows.
Btuh = RA cfm x (mixed air temperature - supply air temperature) x 1.08
= 450 x (70-55) x 1.08
= 7,290 Btuh (cooling)
Now let’s look at estimated heating thermal energy savings through reducing or eliminating the reheat energy through the use of lab space RA.
Btuh = heated air cfm x (supply air temperature - mixed air temperature ) x 1.08
= 450 x (80-68) x 1.08
= 5,832 Buth (heating)
ComparisonsTable 1 shows the comparison between a conventional VAV lab HVAC system with one central large VAV/AHU, and the proposed four small VAV makeup air AHUs, with one AHU dedicated for each floor and one VAV/FCU dedicated for each lab. We assume that all other non-lab spaces in the building are served by separate AHUs. Estimated cost per FCU, installed and operating is provided.
- FCU purchase and installation: $750 (including a factory mounted and wired VSD)
- Chilled water, hot water, and condensate waste piping: $2,000
- Ductwork connections to FCU and R.A.VAV box: $1,000
- Modify lab DDC controls to include FCU: $500
- Electrical service to FCU: $300
Total = $4,550/VAV/FCU, installed and operational
Total systems savings Table 1:
- Item 7: $7,000 -$1,500 = $5,500 savings in supports
- Item 8: $60,000 -$24,000 = $36,000 savings in purchase of units
- Item 9: (10 hp x 0.746 x $0.083 per kWh x 8760)/0.92 = $5,896 per year electrical energy savings because of lower motor hp
- Item 10: $35,000 -$30,000 = $5,000 savings in DDC
- Item 11: $60,000 -$30,000 = $30,000 savings in supply ductwork
- Item 12: $24,000 -$20,000 = $4,000 savings in piping connections
- Item 13: $6,000-$3,000 = $3,000 savings in VSDS cost
- Item 14: $5,000 -$2,000 = $3,000 savings in maintenance cost
- Item 15: $6,000-$2000 = $4,000 savings in assembling AHUs.
Table 2 shows a summary of the comparison results for a 30-yr period. The result is a surprisingly favorable approach in using the FCU system vs. the one centralized VAV/AHU system. Comparison analysis is based on 152,440-sq-ft flab floor area, 457 FCUs, and 1,440 tons of cooling.
ConclusionIn order to perform a satisfactory laboratory HVAC design for your client and an enhancement to the lab VAV system, always seek the idea of several design options, especially if you are not initially sure of the final costs for these options.
- Design one system and make it the base bid, then provide a complete design for the other system and call it “a deduct alternate.” Therefore, the base bid and the deduct alternate should involve two different floor plans, sections, schedule details, and specifications. This way, the mechanical contractor can clearly follow the installation of either design.
- In specifications, create a new section of specifications, and call it “Laboratory HVAC System and Related Controls.” Specify that the fumehood controls supplier is to be the laboratory HVAC contractor who will hire the sheet metal contractor as his sub- contractor; however, the full responsibility will still be by the laboratory HVAC contractor. In this section of the specifications, include the following responsibilities for the lab HVAC contractor:
- Supervise the installation of the VAV boxes by the sheet metal contractor.
- Furnish the VAV boxes and their related controls.
- Furnish factory insulated supply VAV boxes, including the reheat coil section if any.
- Furnish, install and wire the fume hoods controllers .
- Furnish and install all electrical work related to the lab space HVAC system. They may hire the electrical subcontractor to do this work, but the lab HVAC contractor will still be responsible for it.
- Provide check, test, start, air balancing and water balancing inside each lab space, including lab pressurization, testing, and confirmation of fumehoods’ face velocity at different sash heights and in accordance with ASHRAE 110-2005.
- Coordinate work with laboratory DDC contractor to confirm the proper operation of the automatic pressurization of the lab space.
- Furnish, install and calibrate the lab pressurization indicating gauges which are normally mounted above the door. Specify that visualization of the lab pressurization indicator is from both sides, corridor and lab.
- Confirm the constant pressurization of the corridor. Coordinate with the project air balancing and water balancing contractor. The air balancing contractor is to perform balancing work outside the laboratory.
- Try to select a laboratory HVAC contractor who also provides fume hood exhaust fans. This way, the complete lab system becomes one entity’s responsibility.
- A lab HVAC contractor is to be accompanied by the client’s OSHA representative and commissioning agent to provide a label on each hood, signed and dated by the OSHA industrial hygienist, indicating that hoods meet the fume hood velocity requirements (refer to ASHRAE 110-2005).
- Include commissioning work as part of the lab HVAC specifications.
- Where a reheat system is required, design a dedicated boiler system for this purpose, independent of the building winter heating system. ES
- Enviro-Airo Fan Coil Unit Selection catalog, 2003.
- Ottaviano Mechanical Estimating Manual, with related updates, 1983.
- BSRB study by BR-PLUS USA -1 0101.
- Trane Air Handlers catalog/website, August 2005.
- Lab Design News, June 2005.
- DDC Handbook, Siemens Technologies, September 1, 2004.
- ASHRAE, “Simplicity For Dependable Performance And Assured Indoor Air Quality, ASHRAE, Atlanta, January 1998.