HVAC Vacuum Pump Manifold & Chambers
Vacuum pumps that are used in the HVAC trade offer an economical path for
medium vacuum experiments. This article details a simple manifold and chamber
options for a simple vacuum system that is suitable for the hobbyist and educator.
This is an updated version of an article that appeared in Volume 7, Number 1 of the Bell Jar.
IntroductionThis article describes a simple and relatively inexpensive bell jar type system that is suitable for hobbyists as well as middle and high school classroom use. It is based on a two-stage refrigeration service vacuum pump and a readily available plastic belljar/baseplate assembly. With a modest complement of hardware store plus some specialty items, the pump and bell jar can be integrated into a flexible system which also includes a rudimentary provision for pressure control. I have built a number of these systems for the American Vacuum Society's education outreach program and the results have been good. An example of one of these is shown in the photograph below. The pump is a JB Industries DV-85N rated at 3 cfm.
Pump and Manifold
I have used any number of the pumps that are offered to the refrigeration service trade. In years past two major manufacturers were JB Industries and Robinair. These manufacturers still have a strong presence but there is now a plethora of Chinese import pumps. These vary wildly in quality and ruggedness so as a rule I'd recommend the USA built pumps, especially Robinair's 2 stage 6 cfm pump model 15600. The remainder of this article will use the Robinair as the basis for the platform. (My pump is an older model in red, the newer Robinair "CoolTech" pumps are blue.) The pump has a rated base pressure of 20 milliTorr and with a tight and clean system this is readily achieved.
The pump comes with a tee inlet fitting equipped with 1/2" and 1/4 inch male SAE flare fittings (MFL). The 1/2 inch fitting is vertical and provides a good conductance path to a vertically oriented manifold. The first task is to adapt the flare to 1/4-inch pipe thread and this is done using a 1/2 female flare adapter to 1/4" male NPT. The flare connection can be sealed either with a copper flare gasket or with an o-ring that is inserted into the female flare fitting.
The manifold, as shown below, has three primary components that are connected with a 1/4" brass cross: a KF16 flange for the attachment of a gauge, a 1/8" needle valve with 1/4" hose barb for gas inlet control and a KF40 flange for the chamber connection. The 1/4 MFL fitting on the pump's inlet manifold is used with the provided o-ring sealed cap as the vent valve. All threads are sealed with "5 minute" epoxy. A picture of the completed manifold is shown below.
Adapting a Plastic Bell Jar
Nalge Company manufactures a low cost vacuum bell jar and base plate under their Nalgene brand. These are available from a variety of science supply houses for about $100. Edmund Scientific Co. has this item as catalog number 3071338. A drawback of this simple chamber is the little (low conductance) side port that is used for evacuation. I have developed a modification of the baseplate as shown in the figure below.
Start by drilling a 9/16" hole in the exact center of the baseplate (this is easy from the underside). I use an auger bit in a hand brace - the plastic is soft and cuts easily. Tap the hole (3/8 NPT) from the bottom and only run the tap deep enough such that the nipple is just a bit loose when the end of the nipple is flush with the top of the plate. Coat the threads of the nipple with clear epoxy and screw the nipple into the hole. Then feed more clear epoxy into the area around the nipple. Use toothpick or other thin implement to break bubbles and work the epoxy into the volume. Dont use fast setting epoxy! I also rough up the plastic in this area with a Dremel tool prior to gluing to give the epoxy something to grab to. When the area around the nipple is filled, slide the washer over the nipple, fill the remaining threads with epoxy and install the 3/8 to 1/4 adapter. I strongly recommend doing a dry assemby first if for nothing more than practice. Leave this assembly upside down until the epoxy has cured. As for that tiny side port on the base plate, attach a short length of flexible tubing and pinch it closed with a pinchclamp.
Below are pictures of the completed baseplate and the belljar and baseplate mounted on the manifold and pump.
The plastic used in the Nalge product is gassy so only modest base pressures will be achievable. This should be in the 100-150 milliTorr range.Gauging
The system shown at the top of the page has a Bourdon gauge for measuring pressure. The Bourdon is a standard piece of equipment for the AVS's high school outreach program but is of limited use in many applications. Elsewhere on this site and in the compilations are details on commercial and homemade thermal conductivity gauges i.e. thermocouple and Pirani. These will operate over the normal operating range of this system (10 milliTorr to a few Torr). The commercial gauge tubes are generally supplied with either 1/8" NPT or KF16 ports. For the pipe thread gauges a KF16 to 1/8 female NPT adapter flange will be required (along with clamp and center ring). KF16 ported gauge tubes will connect to the manifold directly.Operating Procedure
Read the manufacturers' manuals and precautions as supplied with the pump and the chamber. Place the rubber gasket on the bell jar base plate. Ensure that it is flat and centered between the molded ribs. Place the bell jar on the gasket, centering it.
Pumpdown:Ensure that the valves are in the following positions:
- Pump isolation valve (1/4 turn valve on pump body): Open (handle vertical)
- Needle valve: Closed
- Main vent (threaded O-ring sealed cap at the pump inlet): Closed
- Baseplate vent (pinched tube): Closed
Turn on the pump and monitor the pumpdown via a gauge. At about 1 Torr the sound from the pump will change from a gurgling sound to a clicking sound. If you are pumping moist air, open the gas ballast valve to prevent water from condensing in the pump. Turn off the gas ballast when the moisture has been dealt with in order to get the best base pressure.Venting:
Turning the pump off without isolating the pump or quickly (a second or two or less) venting will result in pump oil backing up through the manifold into the chamber. The best method is to:
- Close the pump isolation valve while the pump is operating.
- Vent the chamber and manifold.
- When the chamber is at atmospheric pressure, turn off the pump and open the isolation valve.Do not stop and restart the pump while under vacuum. This puts a high load on the pump. If the chamber is to be opened and repumped, it is not necessary to turn the pump off. Just control the pumping action by opening and closing the isolation valve.Pressure Control
The chamber may be set to any pressure from full vacuum to just below atmosphere by using the needle valve (lower pressures) or the needle valve and isolation valve (higher pressures). At lower pressures, simply open the needle valve to raise the pressure. You will note that even with the needle valve fully open, the pressure remains fairly low. Further increases may be obtained by throttling the pump by partially closing the isolation valve. The needle valve may be used to admit other gases into the system. Under no circumstances should the system be used with flammable, corrosive, toxic or oxidizing gases.
Adapter for Glass Chambers
The standard KF40 flange permits a wide variety of apparatus to be attached to the manifold. The glass tubes offered elsewhere on this site, for example, are examples of the types of apparatus that can be used with this pumping system. To attach glass tubing a compression adapter is required. I have standardized on a 1-3/8 inch coupling as brass couplings in this size will mate directly to a KF40 x 1.625" braze flange. Rather than brazing I use tin-silver solder. This flows easily, is strong and is also easy to rework, should that be desired. A coupling assembly is shown below along with a complete assembly with glass discharge tube.
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©2008 Stephen P Hansen, the Bell Jar