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Choosing a Diaphragm Pump for a Turbo.

As the requirement for oil-free vacuum pumping spreads to more industrial areas, diaphragm pumps are being looked to increasingly as the roughing/backing pump of choice for high-vacuum applications where turbo or turbodrag pumps are used. Although the requirements for matching a diaphragm pump to a turbo apply to all backing pumps, there are a few considerations specific to diaphragm pumps that need to be kept in mind.

A diaphragm pump works by continuously moving a flexible elastomer diaphragm so as to expand and contract an internal space fitted with inlet and exhaust valves. As the diaphragm is pulled down, the volume of the pump head's internal space increases. This expansion causes the pressure inside the pump head to fall below the inlet and exhaust pressures. The higher pressure at the inlet forces the inlet valve to open, while the higher pressure at the exhaust holds the exhaust valve closed. Because of the higher pressure at the inlet, gas from the inlet enters the pump head's internal space.

As the diaphragm is pushed up, the gas inside the pump head's internal space becomes compressed, which results in the pressure increasing until it becomes higher than the inlet and exhaust pressures. The higher pressure inside the pump head's internal space closes the inlet valve and opens the exhaust valve, allowing some of the gas inside the pump head to escape. This expansion/compression cycling continues until the pressure at the inlet becomes so low that the up-stroke's contractive effect cannot compress the gas to a high enough pressure to open the exhaust valve against atmospheric pressure. At this point, the pump has reached its ultimate pressure where performance stalls.

In a simplified sense, the ultimate pressure of a single-head diaphragm pump can be traced to the pressure differential between the inlet and the exhaust. If a lower inlet pressure is required, the most obvious way to achieve this is to lower the exhaust pressure. This may be done by placing a second diaphragm head in series with the first. The addition of further heads in series will generally produce even lower inlet pressures.

There are three things to keep in mind when matching a diaphragm pump to a turbo. First, check the specifications for the turbo to determine its foreline tolerance. This is the maximum pressure at the pump's foreline that will ensure full operational performance. The ultimate pressure of the diaphragm pump you choose needs to be below this value with a sufficient safety margin for slightly degraded performance or unusually high gas loads. If, for example, the foreline tolerance is listed as 30 torr, you should a choose a diaphragm pump that has an ultimate pressure below 25 torr.

The second thing you need to think about is choosing a pumping speed that will give the diaphragm pump at least the same throughput as the turbo it backs. Pumping speed is the rate at which gas is drawn through a pump when it is operating. Accordingly, pumps are usually rated as volume/time, such as [ft.sup.3]/min (CFM).

What you are looking for is the pumping speed of the backing pump that will maintain the backing pressure at or below the foreline tolerance level. You find that speed by recognizing that the turbo and the diaphragm pump must handle the same gas load. So you need to perform a simple calculation, remembering that Gas Load = Pumping Speed x Pressure.

For the sake of argument, let's suppose that the turbo has a pumping speed of 200 L/sec, that its foreline tolerance is 30 torr, and that the process pressure is [10.sup.-5] torr. That means that the gas load handled by the turbo is 200 x [10.sup.-5] torr L/sec. What you want is the pumping speed of the diaphragm pump that will handle that gas load at a foreline pressure of 30 torr.

Solving the gas load equation for pumping speed gives Pumping Speed = Gas Load/Pressure. So Pumping Speed = 200 x [10.sup.-5]/30, or 6.7 x [10.sup.-5] L/sec. To convert that to CFM, multiply by 2.12. The pumping speed then becomes 6.7 x [10.sup.-5] x 2.12, or 0.00014 CFM.

The third thing you need to recognize is that the pumping speed we've just calculated represents the absolute minimum speed, which in many cases will not meet all of your requirements. Remember that the diaphragm pump will rough down the chamber alone. So you need to choose a diaphragm pump whose pumping speed is appropriate for the volume being pumped, to ensure that it doesn't take forever to pump down the chamber to the point where the turbo can be turned on.

This article was based on information found on the Barodyn page of the

Danielson Vacuum Products Web site at www.danvac.com.
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Title Annotation:pump turbines
Author:Comello, Vic
Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Feb 1, 1999
Words:813
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