A vacuum pump is a device which removes air and other gases from a closed or restricted space. The result of this removal is to create a pressure lower than the surrounding atmosphere.
Vacuum pumps are used to improve the efficiency of steam heating systems in many ways. The most important consideration is the rapid and efficient removal of air. Air actually acts as an insulation. It inhibits the flow of steam and reduces its heat transfer capabilities. One solution to the problem of air in the system is to increase the steam pressure and thus "force" the system, however, this results in higher temperature steam, delayed warm up, uneven steam distribution and wasted fuel. The removal of air with an efficient mechanical vacuum pump solves this air problem in a very economical manner.
Air, oxygen and carbon dioxide, when dissolved in the condensate, cause oxidation and form carbonic acid, which is very corrosive to boiler tubes, pipes and heat transfer surfaces. Sometimes cascading baffles or the turbulent scrubbing action of a liquid ring vacuum pump aid in the removal of these corrosive gases. These features are not usually considered a substitute for chemical treatment.
It was mentioned previously that the result of air removal from a closed or restricted space was a pressure lower than the surrounding atmosphere. In a steam heating system, this lower pressure or "vacuum" results in a greater pressure differential through the system. For example, with 2 psi steam pressure and a return line vacuum of 10"Hg, the total pressure differential is about 7 psi. Without this return line vacuum, it would be necessary to increase steam main pressure to 7 psig, to have the same total pressure differential. This pressure differential is frequently needed to ensure proper drainage from steam traps. Systems originally designed for vacuum usually have smaller return line piping than would be used in an equivalent gravity return system.
It is a characteristic of water that as the surrounding pressure is lowered its boiling temperature is also lowered. For example, at sea level and atmospheric pressure, water boils at 212¼F. At 15"Hg vacuum, water boils at 180¼F. If the return line vacuum is carried throughout the system back of the boiler, it is possible to make steam at this lower temperature. Systems operating in this manner are often referred to as "sub-atmospheric systems" and frequently employ a variable vacuum controller operated by an outdoor thermostat. During mild weather, the system operates on very low temperature steam, and the vacuum level can by as high as 22"Hg. It is only during the most severe weather that the steam is needed at a positive pressure. If a conversion to this type is being considered the thermostatic steam trap elements may need to be changed to remain closed at these lowered steam temperatures.
There seems to be a common misconception among operators of low pressure steam heating systems that the purpose of the vacuum is "to draw the condensate back". In a properly designed system, the condensate flows by gravity to the receiver, with one exception. There may be times when the vacuum pump is called upon to lift condensate. This may be in a remote location and is often found when remodeling work or additions have been added to the building. More frequently, this lift is taken from a low level accumulator tank to the condensate receiver at a higher level. Whenever lifts are present, additional air capacity from the vacuum pump is needed in order to handle this lift in addition to the vacuum pump's primary job of air removal. Without sufficient vacuum in the return piping to lift the condensate, water hammer is certain to occur. This pounding can split pipes and damage trap mechanisms.
Vacuum pumps are sized according to their capacity to move a given quantity of air, during a certain period of time at a given vacuum reading. This is expressed as cubic feet per minute (cfm). The vacuum may be 5 " Hg, 10 "Hg, 15 "Hg or more. For steam heating applications, vacuum pumps may be as small as 2 cfm to several hundred cfm for very large installations. Vacuum pumps are usually sized according to the EDR (Equivalent Direct Radiation) rating of the job, yet these recommendations vary widely among the various equipment manufacturers, from a low of 1/3 cfm per 1,000 square feet EDR to as much as 2 CFM per 1000 square feet EDR for simplex vacuum pump installations on sub-atmospheric systems. It is important to remember that over the life of the heating system, the load on the vacuum pump tends to increase due to minute air leaks, and steam traps in need of service. So it is most important to consider the cfm rating of the vacuum pump and to choose the largest pump that is practical to install. In a tight system, the larger vacuum pump is not necessarily more expensive to operate, since with its greater capacity, it will operate for a shorter period of time to cycle between the settings of the vacuum switch, The usual recommendation for a low vacuum system (10 "Hg max) is one cfm per 1000 sq. ft. EDR. The effect of air leakage and vacuum can be expressed with the following example. One small air leak, 1/8" in diameter will require a flow rate of approximately 3.2 cfm to maintain a vacuum of 8"Hg. Several such leaks can have a drastic effect on the performance of a system with a marginally sized vacuum pump. Another important consideration is the efficiency of the vacuum pump; i.e., cfm per horsepower. With some designs this is in the range of 6 or 7 cfm/hp, while others are capable of moving 15 or 16 cfm/hp. Given these considerations, it is recommended that a vacuum pump be chosen having a reserve of cfm capacity in combination with the lowest horsepower possible for that cfm rating.
There is one further consideration when choosing a vacuum heating pump and that is its "simultaneous capacity". In other words, the capacity when pumping both air and water at the same time. Some designs will have a lower "simultaneous" capacity than their full rated capacity. These units usually employ a water jet type of vacuum producer in which the same centrifugal pump handles both condensate return and motive water for the jet. The flow of water from the discharge of the centrifugal pump is controlled by a float operated control valve. At full rated air capacity this valve is sending all the centrifugal pump discharge through the jet. However, when returning condensate, a smaller amount of water is flowing through the jet resulting in reduced air capacity. Units having separate pumps for vacuum and condensate return will have the same "simultaneous" and "full-rated" capacity and are recommended as the most efficient choice Steam continues to be a popular heat transfer medium in many areas. With the efficient operation of these systems, the result in energy conservation will offset increasing fuel costs.
The vacuum pump has only one moving part -- a balanced rotor that runs without any metal-tometal rubbing contact. Such simplicity is possible because all functions of mechanical pistons or vanes are actually performed by a rotating band of water which serves as a liquid compressant. While power to keep it rotating is transmitted by the rotor, this cylindrical ring of water tends to center itself in the cylindrical body of the compressor. In the circular love design shown here, rotor axis is offset from body axis. As the schematic diagram shows, water almost fills then partly empties each rotor chamber during a single revolution. That sets up the piston action. The air inlet on the vacuum side and the air discharge to atmosphere are separated by means of ported openings, either in a stationary inner cone as shown or in the headplate at the side of the rotor.
Figure 3: Liquid Ring Pump Diagram