When considering a vacuum pump there are several different things to consider. One of the things that you will want to consider is the type of device that runs your vacuum pump. As with everything there are pros and cons. Let's take a look at some of the different devices you can choose from to run your vacuum pump.
One of the devices is an ejector. This is one of the simplest and commonly used as a vacuum producer. The basic process of the ejector is that it converges pressure energy of a dynamic fluid into velocity energy as it flows through a typical small nozzle. Ejectors use many different types of motive fluid. Steam is one of the most common fluids. Some of the pros of an ejector for a vacuum pump are:
1. It is a simple design
2. It has no moving parts which makes it so that there is not much wear
3. Has the option of being fabricated from any type of metal. It can even be fabricated of various types of plastics.
4. Using an ejector for a vacuum pump tend to be one of the least expensive ways to make a vacuum device
5. Can offer the largest throughput capacity of any type of vacuum producing device
6. It is easy to start or shutdown the vacuum pump. There is no special way to do it so it is easy for anyone.
7. Due to not having moving parts and being a simple design it is easy to repair and very low maintenance.
All of these things sound great, but there are some disadvantages for an ejector as a device for a vacuum pump. Some of those disadvantages are:
1. There is a requirement for the pressurized motion fluid
2. It is inevitable that contamination of the dynamic fluid will happen
3. It can be noisy
4. With the noise it could require discharge silencers or sound insulation
5. Most of the time there is a need for a cooling liquid source
Another option for a vacuum pump device is a liquid ring pump. This vacuum pump device is a multi-bladed impeller with a shaft that is mounted so that it can be an eccentric to the ring of liquid. Basically, as the blades rotate toward the discharge side of the pump the pockets decrease in size and the evacuated gas is compressed, enabling its discharge.
Some of the advantages of a liquid ring pump for a vacuum pump are:
1. Simpler design than most vacuum pumps
2. It employs only one rotating assembly
3. It can be constructed from any castable metal
4. It has minimal noise and vibration
5. It has very little increase in the temperature of the discharged gas
6. Can handle condensable loads
7. There is no damage from liquid or small particulates entrained in the process fluid
8. The maintenance is very simple compared to other vacuum pumps
9. If the pump needs to be rebuilt it is simple
10. It has a slow rotational speed which gives it a longer operating life
11. Can easily be started and stopped over and over
12. Can use any type of liquid for the sealant fluid
Like everything, there are cons to the liquid ring pump. Here are some of the cons:
1. It is inevitable that there will be a mixing of the evacuated gas with the sealing fluid
2. High power requirement in order to form and maintain the liquid ring
3. Due to the high power requirement it results in a larger motor than for other types of vacuum pumps
4. There is a risk of cavitations. This means that it requires a portion of the process load to be noncondensable under the pump operating conditions.
5. The achievable vacuum is limited by the vapor pressure of a sealant fluid at the operating temperature.
Another option for a vacuum pump is a dry pump. A dry pump does not need any working fluids. There are three different types of vacuum pumps available. They are: hook and claw, screw and lobe types. These types of vacuum pumps work by two different types of mechanisms. There is either volumetric reduction or the mixing of low-pressure gas with higher-pressure discharge gas. With a dry pump it normally runs hot, because there is no liquid to absorb the heat of compression.
Some of the advantages for a dry-running pump are:
1. No contamination of evacuated gas which allows for the gas to be recovered readily
2. The pump can be built of standard, inexpensive cast iron. This is the case due to the lack of condensation.
3. It can discharge to the atmosphere
Some of the disadvantages of a dry running vacuum pump are:
1. Can not handle particulates
2. Can not handle large amounts of liquid
3. Might require a silencer
4. Due to it running hot it can discharge gases at high temperatures. Sometimes the temperatures can run as high as 600 degrees.
5. Most case it can be difficult to repair or rebuild
6. Due to the high temperatures it may require a gas purge for cooling to protect the bearings and seals from the process gas
Another option for a vacuum pump is a once through oil pump. It is commonly known as OTO for short. It is a vacuum pump that is a sliding vane type that uses once through oil to seal clearances and lubricating moving parts.
Some of the positives of an OTO vacuum pump are:
1. Can handle acidic or otherwise corrosive vapors
2. Can be built of cast iron
3. Has minimal vibration
4. Can handle high inlet temperature
Some of the negatives of an OTO vacuum pump are:
1. Requires the use of immediately removing the liquids to prevent liquid from hiring the vanes.
2. There can be contamination of the discharged process gas by the oil
3. Can not handle particulates
4. It can be cost to repair or rebuild
There is one other option for a vacuum pump device and that is a rotary piston model. The piston is attached to a cam that is mounted to the main bore of the pump cylinder.
There are several different pros to a rotary piston model:
1. Minimal vibration due to it having a balanced configuration
2. It has a rugged design so that it lends to a long life
3. Can handle small particulates
Some of the cons to a rotary piston vacuum pump model are:
1. Can not handle liquids
2. The condensable vapors are not easily handled
3. The discharged gas is contaminated with oil
4. Some of the designs can be noisy
With all of this information in mind you will want to weigh out the options and the specifics you are looking for that best meets your needs.