Tech Talk: Air Capacity and Consumption – Choosing Electronic to Pneumatic Transducer

When it comes to selecting an electronic to pneumatic (E/P) transducer, the type of input and the pressure range of the output are the most common selection criteria considered. There are a couple of other specifications that are worth reviewing to select the best transducer for the job. These additional specifications are Air Consumption and Air Capacity.

Air Consumption

An E/P transducer is designed to vary air pressure to a pneumatic load like an actuator based on a varying electronic signal from some sort of controller. The source of air pressure used by E/P transducers, referred to as “main air,” originates at an air compressor and makes its way to pneumatic devices throughout the building via tubing.

Different types of E/P transducers use different techniques to vary the air pressure to their output or branch line. Some transducers incorporate a design that is like pneumatic thermostats and controllers with a nozzle and flapper. These transducers do allow a small amount of air to escape to the atmosphere. This small amount of air is called Air Consumption and is normally specified in units of scfm (standard cubic feet per minute).

Actual air consumption can be affected by the amount of main air pressure but is normally specified as an average value such as 0.008 scfm. Since there is a given amount of air available in the main air lines and the air compressor’s storage tank, the number of pneumatic devices that consume air affect how often the air compressor runs as well as the size of its storage tank.

If an available pneumatic air supply does not support numerous air consuming E/P transducers, there is another type of transducer design that uses two solenoid valves to increase and decrease pressure in the branch line. This type of design does not constantly bleed air, other than what is exhausted from the branch line during normal operation. In other words, this type of transducer does not consume air and is not normally considered when sizing an air compressor. The other option is to increase the available air supply with an additional or larger air compressor.

Air Capacity

A pneumatic transducer’s air capacity specification defines how much air volume can flow through the transducer in each amount of time. Air capacity is usually specified in units of scim (standard cubic inches per minute), or scfm, at  main air pressure, with a minimal pressure drop across the transducer. A typical air capacity specification for a pneumatic transducer might be 500 scim at 20 psi with a 1 psi pressure drop. In general, air capacity relates to how quickly an E/P transducer will pressurize a given volume connected to its output.

Calculating the exact time, it would take an E/P transducer to completely fill an actuator to full pressure is difficult. On the surface dividing the total volume connected to the E/P transducer’s output (including the actuator and tubing) by the E/P transducer’s air capacity would provide an easy answer. However, there are other forces at work. The force of the internal actuator spring as well as the load connected to the actuator act against the pressure building in the actuator. This opposing force slows the rate at which pressure builds in the actuator. For example, with an 8-13 psi spring range actuator, an E/P transducer will fill the empty volume of the actuator and tubing relatively quickly until 8 psi is reached. At this point, the opposing force of the actuator’s spring and load is encountered causing a noticeably slower increase in pressure. In addition, the rate of pressure increase will slow as the pressure in the branch line approaches the main air pressure.

Depressurizing or exhausting the air in the branch line through the E/P transducer is another story. Depressurizing a given volume of air takes longer than it does to pressurize. When building up pressure, the transducer allows main air to flow into the branch line. In other words, there is a constant 20 psi source of pressure pushing air into the tubing and actuator connected to the transducer output. When depressurizing, the E/P transducer opens the branch line to the atmosphere allowing air to escape from the actuator and tubing. In other words, the built-up pressure in the branch, with a little help from the actuator spring, is what pushes the air out of the branch into the atmosphere. Obviously, as the air is exhausted, the branch pressure will decrease, and therefore, there is less pressure to push the air into the atmosphere. With less and less pressure, the rate at which air is exhausted becomes slower and slower.

While selecting a transducer with adequate air capacity is important, knowing the precise fill and exhaust times is not critical for most HVAC applications. As a rule, our UCP-242 and UCP-422 transducers are suited for small to medium pneumatic actuators. For medium to large actuators, use the UCP-822.

Summary

Selecting the proper E/P transducer for an application involves more than choosing the input signal and output pressure range. Air consumption is an important consideration when designing a new job or planning a retrofit, especially when numerous pneumatic transducers that consume air are involved. While not an exact science, selecting an E/P transducer with adequate air capacity will improve the response and performance of the control loop. Kele stocks a complete selection of competitive electronic to pneumatic transducers that will meet the requirements of most any application. Give Kele a call to discuss your next electronic to pneumatic transducer requirement.

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