Heating Gas Lines: Heat Tape or Flexible Heaters?
If you need to deliver gas into a process chamber in a vapor state, then you need to heat your delivery line and components to prevent the vapor from condensing back into a liquid state prior to reaching the chamber.
The conventional and lowest cost method is to use what is commonly called “heat tape,” which is not really tape at all; but is typically a fairly stiff, encapsulated, resistance heated set of wires that you run along your delivery line, either in a spiral or straight as shown below, and then cover with a suitable insulating material. As you might imagine, neither of these methods result in uniform pipe heating. One way around this issue is to put a layer of aluminum (conductive) tape over the piping before installing the insulation. While this does increase the temperature uniformity, it is time consuming and often painful – literally – because aluminum tape cuts are NOT fun. Thus, beware of the hassle and maintenance cost in replacing failed heaters. So, if you have a very simple delivery line and do not need to worry too much about temperature uniformity, heat tape may work for you.
If, on the other hand, your delivery line takes a circuitous route, or contains valves, filters, MFCs, etc., you will find heat tape to be ineffective because you will almost certainly encounter cool or cold spots in the hard to heat corners and crevices and in large surface area components. Even one cool spot can cause condensation issues which will wreak havoc on your process by way of excess particulates, inconsistent vapor delivery or even complete delivery blockage.
A good solution for achieving uniform coverage is to use flexible heaters that form fit to your delivery line tubing and components. These are typically made using a nichrome wire mesh which is encased between two thin layers of silicon rubber. An outer shell of silicon rubber foam is either fused on, or added separately. Two common examples are offered by Watlow. One is the "Stretch to Length" or “S-T-L” product line, and the other is the formed Gas Line Heater product line. Which one to choose depends on the length and level of complexity of your gas line, and the expected frequency of service.
S-T-L heaters are extremely versatile, reasonable in cost, and provide good heat uniformity. You can wrap the heater such that there are only small or no gaps, or, you can stretch the heaters out along the piping, particularly on a straight section, to save some cost. The only downside is installation consistency, as they are difficult to install the exact same way repeatedly. They are most appropriate for one-off or after-market gas line installations.
Formed Gas Line Heaters are best for OEMs and/or if the gas line is going to be removed and reinstalled in the field. These heaters are formed specifically to fit various lengths of straight pipe, valves, filters, fittings, etc., and include a layer of silicon foam insulation. They are made with a seam (like a hot dog bun) that serves as a parting line that allows for easy installation /removal/replacement in the field. Each heater is designed to be a part of a kit of parts that will connected in series. The electrical connections are actually wired in parallel so that if you lose a single heater the rest of the system continues to operate and provide heat. The disadvantage other than the added cost is that the piping of the system needs to be thought about prior to building. Because low volume custom lengths are not economically viable, Watlow gas line heaters only come in specific lengths, thus you need to keep the standard lengths in mind when designing your gas line in order to minimize areas that require a custom solution.
Now you’ve selected the type of heaters you need. What about control? For any of these heater solutions you should use a good auto-tuning PID control with a fast output circuit firing an SSR (Solid State Relay) to control power to the heaters. One solution we like is the Watlow PM Series temperature controls because they offer a controller with an FM Approved High (or Low) Temperature Safety Limit built-in. By simply adding a second thermocouple and a mechanical relay (if your load is big enough), you will have system safety built right in - it can be a cost and space saver if you are considering or require a safety limit.
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