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TRAMS: A New Tracer Gas Airflow Measurement System

The technologies currently available to measure airflow rates in duct systems require careful use and substantial time to produce accurate measurements. Traditional measurement systems use Pitot-static tubes or hot-wire or other anemometers to measure velocities at several locations in the cross section of an airstream. It very difficult to accurately measure airflow rates using these methods because of problems such as large spatial variation in air velocities and air velocities that are so low that they approach the detection limit of the anemometers.

Image of Injection module

Figure 1. The whip injector; note the small, light weight CO2 cylinder

If a tracer gas is injected into a duct at a known rate and the true average concentration of tracer gas upstream and downstream of the injection plane can be determined, the flow rate is calculated from a simple mass-balance equation. However, the application of tracer-gas methods has been limited because of poor mixing of the tracer gas in the airstream, the high cost of the systems, and the substantial time required to perform measurements.

The Indoor Environment Department (IED) of the Environmental Energy Technologies Division (EETD) at Lawrence Berkeley National Laboratory (Berkeley Lab) has invented a new tracer-gas measurement method that overcomes these limitations. The Tracer Gas Airflow Measurement System (TRAMS) will have a range of applications, including balancing of airflows in heating, ventilation, air conditioning (HVAC) systems in commercial buildings; diagnosis of HVAC systems for identification of energy-efficiency opportunities; and calibration of in-place airflow measurement stations.

Description

Closed blades on the injection module Open blades of the injection module

Figure 2. The mixing fan, with blades folded for insertion in the duct (top) and with blades unfolded

TRAMS uses carbon dioxide (CO2) as its tracer gas for a number of reasons: CO2 is inexpensive, non toxic, and widely available in cylinders, and CO2 analyzers are much less expensive than the analyzers used for conventional tracer gases, such as sulfur hexafluoride. To ensure that injected CO2 mixes effectively in the airstream, TRAMS uses "whip" injectors (Figure 1). These injectors are very small, flexible tubes that are easily installed in duct systems and emit CO2 at high velocity, which causes them to whip around rapidly in the airstream, dispersing the tracer gas. Easily insertable fans (Figure 2) with fold-up blades and externally mounted motors help mix the tracer gas with the airstream. The soaker-hose sampling system for the CO2 analyzer allows a user to easily collect samples.

Advantages

The major advantages of the TRAMS system are:

  • Its unique mixing and sample-averaging techniques effectively mix CO2 in the airstream within two hydraulic diameters of the injection site.
  • TRAMS uses a gas analyzer that costs one tenth the price of most other tracer-gas analyzers.
  • TRAMS does not require a mass-flow controller to regulate the tracer- gas injection, which makes operation simple and minimizes costs.
  • The TRAMS apparatus is easy to install and not intrusive. Each injector requires only a -inch-diameter hole through the duct. Each mixing fan only requires a -inch diameter-hole. Two -inch-diameter holes are required to insert a soaker-hose sampler.
  • TRAMS is versatile. The whip injectors and mixing fan blades can be customized to fit rectangular, circular, or oval ducts of different dimensions. Varying the CO2 injection rate allows measurements of a very broad range of airflow rates.
  • Installation is fast. The mixing fans and whip injectors are held in place on the duct by magnets. No mechanical fasteners are required.
  • Measurement is fast and repeatable. Injection takes about 10 seconds; one measurement process takes minutes.
  • Data processing is computer automated.

Test Results

Figure 3 shows the results of 32 laboratory tests of TRAMS measuring airflows in straight rectangular ducts, with and without a Tee junction upstream of the measurement section. The reference flow meter has a rated accuracy of 0.5 percent. These results indicate that the accuracy of TRAMS is better than two percent.

Chart illustrating TRAMS' accuracy in a laboratory study: x-axis = True (reference) Flow (cfm); y-axis = % Error in Tracer Gas Measurement.

Figure 3. TRAMS' accuracy in a laboratory study

— Duo Wang

For more information, contact:

  • Duo Wang
  • (510) 486-6878; Fax (510) 486-6658
  • Max Sherman
  • (510) 486-4022; Fax (510) 486-6658

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