Functional Description of Gas Boosters

HASKEL air driven Gas Boosters incorporate three basic functions. (1) The air drive which supplies the power for gas compression, (2) the cycling system, which controls the reciprocating (back and forth motion), and (3) the gas section which provides the flow of gas. These three functions work together to provide proper operation. Damage or contamination, affecting any one of the functions, can result in erratic operation and/or compromised performance.

HASKEL air driven Gas Boosters are "ratio" devices using low pressure air against a large piston area to generate force (force= pressure * area). This force is transmitted through a smaller area plunger or piston in the gas section to increase the pressure of a gas by a factor equal to the nominal area ratio between the air drive and the gas piston/plunger. Using this principle, a large, low pressure air flow can be used to generate a smaller higher pressure gas flow.

AIR DRIVE FUNCTION: The cycling system, built into the air drive end caps, provides the continuous reciprocating action of the booster. It is composed of an air piston assembly, two pilot valves, and an air cycling valve. The action of the valve depends on an "unbalance" of areas exposed to the drive pressure and the pilot pressure. With the pilot pressure vented, the air drive pressure works against an area of the valve spool to move it to a position routing the air through a flow tube to the pilot vent end cap. This drives the air piston towards the valve end cap. When the air piston contacts and opens the pilot fill valve in the valve end cap, the pilot chamber is pressurized. This pressure, acting on a larger area of the spool than the drive air, moves the spool to its other position. In this position, the pressure on the air piston is shifted to drive the piston towards the pilot vent end cap. When it reaches the cap, it opens the pilot vent valve, permitting the pilot chamber to vent (through the pilot tube) and shift the spool to its original position. The cycle of pressurizing and venting the pilot chamber continues to repeat automatically as long as air pressure is available and the outlet pressure is below stall pressure.

GAS SECTION FUNCTION: The gas section consists of a gas barrel to contain the pressure, a plunger or piston to move the gas, and check valves to control the flow direction. The size and shape of this section will vary with the size of the air drive, the nominal booster ratio, and the check valve configuration. Its operational mode is basically the same for all gas boosters.

The plunger/piston is mechanically connected to the air drive piston and extends through a sealed separation into the gas section. The reciprocating (back and forth) movement of the plunger/piston alternately increases and decreases the volume in the gas barrel (the difference between the two volumes is the displacement per cycle). When the volume is being increased (suction stroke), the "unswept volume" in the gas section must expand until the residual pressure is lower than the supply pressure, and then the inlet check valve opens and gas enters the gas barrel. When the plunger/piston reverses direction, the inlet check valve closes to trap the gas, and the outlet check valve opens to direct the flow into the downstream system (compression stroke). When the stroke is complete and another suction stroke is started, the outlet check valve closes. This prevents system gas from returning (back flowing) into the gas section. The process is repeated automatically each cycle until a force/balance condition is achieved between the drive force of the air drive and the downstream pressure on the plunger/piston (stall).

The "output flow" is the result of piston/plunger displacement. The "pressure" is generated by system resistance (back pressure), not the total force available. The "excess" force available over that required to generate pressure is used for "cycle rate", and determines the output flow. With no back pressure, all of the energy is used for cycle rate, and it will cycle rapidly. As the system builds in pressure, the excess energy decreases, and the booster will cycle progressively slower until it reaches the "force balance" point and then it will stop (stall), because all of the energy is required to generate the desired pressure.

Stopping the booster traps and holds the downstream pressure. The maximum outlet pressure produced by these boosters is a function of the "nominal ratio", the "maximum compression ratio" and the air drive pressure. For example, an AG-30 (Nominal 30:1 ratio; max compression ratio 25:1) booster with 100 psi air drive and 200 psi supply will develop 3000 psi at the stall condition. Note that if the supply pressure were only 100 psig, the maximum pressure attainable would be about 2800 psig (maximum compression ratio), and the unit would continue to cycle slowly. When the maximum compression ratio is reached, and it is less than the stall pressure, the booster cannot inhale new gas on the suction stroke. This is because the pressure in the "unswept volume" is so high that it cannot expand enough to lower the gas section pressure below supply pressure. The booster then continues to compress and expand the same residual gas and it does not flow into the downstream system.

If the downstream pressure is reduced due to leakage, the use of gas downstream, or if the air drive pressure is increased, the boosting action will re-start automatically because the forces are no longer "balanced". This is an advantage in any application requiring extended periods of constant pressure, because there is no power used at stall as would be required by an electric or hydraulic motor drive that must run continuously.

SYSTEM DESIGN CONSIDERATIONS: The most important factors in maintenance of any mechanical equipment are proper selection and proper system installation. Boosters should be selected to provide the best match for the application (flow rate, pressure rating, gases used, and type of duty, etc.).

The system should provide an inlet screen on both air and gas supply lines to prevent particulate contamination from damaging seals and sealing surface finishes.

Permitting boosters to operate "unloaded" for extended periods of time can result in excessive seal wear and higher maintenance cost, as well as shortening its useful life. Running "unloaded" is equivalent to taking your car out of gear at full throttle...not very good for service life. Throttling the air drive supply during lightly loaded conditions will give better seal life.

Provide an adequate supply of gas to compensate for system usage. With a fixed supply volume, depletion can cause lowering of supply pressure to the point that "maximum compression ratio" and "volumetric efficiency" can affect its ability to recharge the system in a timely manner.

For applications involving "high purity" gases, downstream filtration is recommended since no dynamic seal (one that is moving with respect to the mating part) can be "particle free".

Supply piping for the air drive and gas sections should be at least the size of the connections to obtain the rated catalog performance of the booster. Too small a line size on the air drive supply will result in slower operation and less output. Too small a gas supply line can cause "starvation" (incomplete filling of the gas barrel), and failure to meet the expected flow rates. Too small a downstream line will cause excessive resistance, making the booster think it working against a higher system pressure, and it will slow down accordingly and put out less flow. Where the booster pressure capability is greater than the system design pressure, a relief valve should be installed with adequate flow capacity to prevent over-pressurization.