When the Joule-Thomson effect is in play you may need a heated regulator to keep gas temperature above the dew point. A heated regulator is a pressure-reducing regulator in which system fluid flows over a heated element. A heater cartridge is required.
To ensure buying a heater cartridge in the right power range, calculate the number of watts required (Pw) via:
Pw = Qn Cp ΔT
where Qn is molar flow, Cp is heat capacity of the fluid and ΔT is Joule-Thompson coefficient or the amount of cooling from the Joule-Thomson effect for the particular gas.
In our third regulator application you must vaporize a liquid before it can be sent to a gas chromatograph or other analyzer. This calls for a vaporizing regulator.
The objective of a vaporizing regulator is to instantly flash the entire sample. This requires applying a lot of heat at the precise location of the pressure drop.
Vaporizing regulators are tricky but, if properly sized and installed, can reliably prepare a liquid sample for a gas analyzer.
You must pay close attention to flow. Too great a flow will lead to partial vaporization and liquids passing through the regulator toward the analyzer. Too little flow will prompt vaporization of the liquid sample upstream.
Failure to correctly set up your vaporizing regulator can create considerable time delay. As fluid changes from liquid to gas, volume dramatically increases. The rate of increase depends upon the liquid's molecular weight.
Typically, measured vapor flow after the regulator will exceed liquid flow before the regulator by more than 300 times. For example, a vapor flow of 600 cm3/min may stem from liquid flow of less than 2 cm3/min. Therefore, the liquid will take 25 minutes to travel through 3 m (approximately 10 ft) of 6-mm (¼-in.) tubing. To decrease this time, you must reduce the volume of tubing preceding the regulator. For example, with only 1 ft of 1/8-in. tubing, liquid would reach the regulator in only 30 sec. However, you must add to that time delay in the probe — the narrower the probe, the faster the response.
Another way to attain a faster response is to move the regulator closer to the analyzer with the aid of a second fast loop. Figure 2 shows the regulator located after the fast loop filter with a second liquid fast loop ensuring good liquid flow continues right to the vaporizing regulator. The objective is to minimize slow-moving liquid volume going to a vaporizing regulator.
Tackle Time Delays
A regulator is a critical tool in addressing time delay in an analytical system. By reducing pressure, you reduce the delay. The lower the pressure in a gas system, the faster the response time. In general, the sooner system pressure can be dropped, the better.
Where liquid must be vaporized, make intelligent use of fast loops to keep the liquid moving right up to the vaporizing regulator.
The field station is one place in a complex analytical system where you can significantly reduce time delay. However, this requires a comprehensive approach that scrutinizes all potential causes of delay in the system.
DOUG NORDSTROM is market manager, analytical instrumentation, for Swagelok Co., Solon, Ohio. MIKE ADKINS is general industrial valve product manager for Swagelok in Solon. E-mail them at firstname.lastname@example.org and email@example.com.