Laboratory Design Newsletter 2012 Selected Abstract

Reduce Energy Costs with Standby Laboratory Exhaust Fans

Victor Neuman, Schneider Electric

Abstract

The biggest user of energy in a laboratory is the HVAC system. Few techniques have been applied to saving energy and operating cost in the chemical exhaust portion of the air-conditioning system. The current practice is to have two chemical exhaust fans for each exhaust duct. One is the operating fan running 100 percent and the second is the standby fan, which is off. Significant energy savings can be achieved by running both the operating fan and the standby fan together, each at 50 percent of design flow. Owners can expect paybacks for typical two 50 percent exhaust fan projects to be approximately one year. This quick payback together with the short construction time makes this an exceedingly attractive project.

Two 50 percent exhaust fans save energy by moving down the fan curve. If fans used energy in a linear fashion, there would be no savings in moving from one 100 percent exhaust fan to two 50 percent exhaust fans. But the ideal fan volume curve is based on an exponent of 3. If you reduce the flow in a fan by 10 percent, the power usage is reduced by 27 percent for an ideal fan. Actual manufacturer's fan curves should be used to find actual fan energy savings.

It is highly recommended that a risk assessment study be performed for any operating chemical, biological, or radiological exhaust fan. Such a study evaluates the amount and toxicity of all chemicals and air-borne hazards likely to be found in the exhaust system. A credible spill scenario is constructed and analyzed in terms of dilution rates to receptor sites like doors and windows. The best kind of risk assessment for a chemical exhaust fan includes a wind tunnel study. Two possible worldwide global firms that conduct these are www.cppwind.com and www.rwdi.com.

We are now running both fans and both fans could fail simultaneously. This energy saving strategy seems to eliminate the standby fan or lead-lag fan operation. However, it has been found that it is not reliable to run one fan all the time and to never run the backup fan. As a result, the lead fan and standby fan are exchanged regularly. In fact, in most cases, both fans are given equal run times in alternation. This being the case, the degree of backup and redundancy is equal between lead-lag and running both fans simultaneously.

EXAMPLE:

We will examine a hypothetical large chemical laboratory building, which is used for teaching and research at a Midwestern USA location. As is required by safety standards, the chemical exhaust fans serving laboratory exhaust and chemical hoods normally do not shut-off. Electrical costs average $0.10 US dollars (USD) per kilowatt hour.  

This building is served by two identical Greenheck 44AFSW-41 fans. Originally, one fan was on at 100 percent of design at 60,000 cubic feet per minute (cfm) and a pressure capability of 2.0 inches of water gauge, and the second fan was off on standby.

Originally, the main fan used 70.81 brake horsepower. In our example, the university energy conservation team modifies the building automation system so that both the main and standby fans run simultaneously. Now, instead of one fan at 100 percent of design volume, we have two fans running at 50 percent of design volume.

Using the manufacturer's published fan curves, the new control scheme results in each fan running 30,000 cfm at 2.0 inch water gauge. Each of two fans is 16.2 bhp for a total of 32.4 bhp. This change saves 38.41 bhp, more than half of the original running energy for one exhaust fan is saved by running two fans at 50 percent.

What might be an average cost of making this change? As discussed, it is important to perform a risk assessment of the exhaust fan running at 50 percent flow as it relates to chemical dilution and site geography. It would be better to run a wind tunnel simulation, but assume one step down from that for a manual calculation study costing $17,500 USD.

In our hypothetical university, as is common practice, the variable speed drives have already been installed, one per exhaust fan. If the university staff do the reprogramming of the building automation system, the cost will be minimal. Assume, to be on the safe side that an outside contractor reprograms the building controls for the new operating mode is $10,000 USD.  The resulting cost benefit analysis is:

The base case is 70.81 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $46,274 per year to run one fan at 100 percent volume.

The energy use for two fans at 50 percent volume is 32.4 hp x .746 kw/hp x 8760 hrs x 0.10/kwh = $21,173.

The annual savings is $25,100; the installation costs are $27,500; and the simple payback is 1.1 years.