In the event an existing vacuum pump is being replaced due to age or on-going repair requirements be sure to consider the improved efficiency and technology now available. It is very likely that a smaller horsepower (KW) pump may be utilized. This will result in an immediate reduction of power costs.
Central System or Individual Pumps?
If a new plant or facility is being established the decision must be made as to the desirability of a central system as opposed to installing an individual pump for each machine. Both have advantages and shortcomings.
Central System
A central system offers the distinct advantage of a single location for all related equipment. The pumps themselves, the vacuum receiver, filters, electrical disconnects, switch gear and so forth, are all in one location. This makes for easier maintenance. Oil and filter changes are performed on one or two machines (if redundancy has been provided). The tank can be drained. Coolers can be cleaned. Discharge can easily be routed to a single point if required. Most often the discharge is simply allowed to vent directly to atmosphere. (This practice can be dangerous to the health of anyone inhaling the discharge if hazardous material, chemicals or gases are being drawn into the system as a result of processes, experiments or laboratory procedures). If the a lubricated pump or system is poorly maintained oil vapors or aerosols will collect and condense at the pump or system discharge resulting in a hazmat condition.
There are several other considerations that must be taken into account when prior to installation of a central system. If only one pump is installed, the vacuum system must be completely shut down when performing oil and filter changes. Any repair requiring the shut-down of the pump interrupts all processes utilizing vacuum. If a repair requires a lengthy shutdown will require a rental pump be located and installed.
If additional process or manufacturing equipment is installed a larger pump may be required. This will entail the installation larger wiring, conduit, breakers, fuses and switch gear.
If the volume of the pump is increased a larger piping system may be required. Remember, vacuum is much more susceptible to pressure drops in the piping, caused by friction, than compressed air. With compressed air oftentimes it is possible to increase the pressure at the pump and accept the increased cost of energy. With vacuum, that’s not possible. Remember, a vacuum pump relies on atmospheric pressure to move the air through the pipe to the pump. If the pipe is too small to allow unopposed flow the vacuum pump cannot evacuate the piping. This is especially noticeable where draw-down time is critical to production. Vacuum molding of a heated material onto or into a mold is a perfect example. If the draw-down time is too slow, the material will cool to the point of not forming to the mold.
Speaking of the system, the larger the system, the more susceptible it will be to leaks. Leaks in a vacuum piping system are very difficult to locate. Vacuum makes no easily detectable sound as does compressed air. A vacuum system is usually taken off-line and pressurized in order to locate leaks. This is a major expenditure as it must be performed when there is no production, usually after hours or on a weekend when premium costs are applicable.
System repairs are not usually easily accomplished. Vacuum piping is usually large diameter PVC. While PVC is lighter than most other materials, it still has some weight and is awards. Remember. A twelve to fourteen inch or even larger system is not at all uncommon. It’s not like you can run down to the local hardware store and pick up a joint of 14″ PVC or an elbow or tee. These are usually special order items.
Individual Pumps
While individual pumps installed for each machine results in increased maintenance costs due to each pump requiring oil changes, internal demisters (in the case of lubricated vane pumps) and the routing of the pump discharge to either the exterior of the building or into a collector, there is no danger of energy wasting leaks in an extended piping system. There is also the advantage that only the machine utilizing the individual pump must be shut down during the maintenance cycle, and then only for the time required to perform the maintenance. The oil and filter change can usually be performed during the operators’ lunch break.
Draw-down times are much quicker when the only additional volume is the short connecting pipe from the pump to the vacuum chamber or vacuum chuck on the manufacturing machine.
Breakers, wiring, conduit, switch gear and disconnects must be provided for each pump if the pump is not connected directly to the control panel of the machine it is dedicated to. Also, any required temp switches, vacuum pressure switches or vacuum breakers must be provided for each pump.
As you can see, both systems have their advantages and disadvantages. If there are several machines requiring vacuum and draw-down time is not critical, a central vacuum system might be the most economical in terms of installation and maintenance.
If draw-down time is critical a central system will probably not be the system of choice.
Hybrid Systems
While the next system isn’t actually a hybrid, it is becoming more common. The best of both worlds seems to be small central or modular systems installed throughout a plant. This method consists of serval small vacuum centers providing vacuum for a cluster of machines. This allows maintenance to the vacuum pumps without a complete shutdown. Requirements for additional electrical components and circuits are lessened. Only one pump discharge needs to be dealt with.
Types of Pumps
Here the decision must be made as to how much vacuum in terms of flow and vacuum pressure is required. This will be determined by the application. If you only need a few ACFM at 24″Hg a small desktop unit might be the solution. If you need hundreds of ACFM at 29.9″Hg a much larger unit will be required. If you are not totally familiar with vacuum systems, their sizing methods and the design of a vacuum system, consult someone. If you choose to consult an engineer, demand their credentials as regards vacuum. There are a LOT of individuals that are of the opinion that vacuum is a no-brainer. It is just a matter of flow versus pressure. Nothing could be farther from the truth. A vacuum system is much more difficult to design, size and install than a compressed air system.
Lubricated Rotary Vane
The lubricated rotary vane pump is by far the most common. It is capable of a wide range of vacuum pressures. IT is used in two stage applications where a deep vacuum is required. It can be used as a roughing pump when millitorr vacuum pressure is required. It can be used where much less vacuum is required as well. It will operate efficiently where elevated ambient temperatures are present. This is a the most versatile configuration by far.
Dry Rotary Vane
Next is the dry vane pump. This type is used where no oil is allowed in the system. While there are many in use, they are a limited capability pump. They are very sensitive to high ambient temperatures as they run very hot. They are capable of limited pressures, usually around 24″Hg maximum. They are also very high maintenance. The vanes have a high wear rate and must be inspected periodically to prevent catastrophic failure.
Scroll Pump
The rotary scroll has become fairly prevalent. While small in volume capability, they are a wonderfully quiet pump. Once again, they’re not capable of deep vacuum but they are a reliable pump when maintained properly.
Rotary Screw
The oil flooded rotary screw vacuum pump (actually a modified rotary screw air compressor) is fairly popular. This machine has some limitations. If MUST pull at least 16″ Hg or it will pass oil across the separator element. It will pass a LOT of oil across the separator element. These units utilize a separate oil pump with the attendant seal maintenance requirements. These are available in belt drive as well as direct drive. There is a brand that uses an internal oil pump connected directly to the rotor and housed inside the air end.
Piston Pump
Then there is the old original. The reciprocating or piston pump. This type has been around longest. It is still in production and is indispensable in some applications.
Diaphragm Pump
There is also the diaphragm vacuum pump. Oilless, limited capability and not too common.
Get Knowledgeable Assistance
I cannot stress strongly enough; if you are not thoroughly grounded in vacuum theory and technology, don’t go it alone. Get someone to consult with. Get someone with plenty of experience designing and installing vacuum pumps and systems. Regardless of the cost you will spend many times the cost rectifying mistakes down the road. It might be that you will manage to come up with a functional system but that system might be costing you thousands or tens of thousands of dollars each year due to inefficiencies.
Even if you are simply replacing an older existing pump, get someone familiar with the newer available technology. In most instances the new pumps are significantly more efficient. You can possibly go with a smaller pump, thereby realizing an immediate reduction in the monthly energy bill.