Gas pressure boosting may be used to fill storage cylinders to a higher pressure than the available gas supply, or to provide production gas at pressure higher than line pressure.
(1) Breathing gas blending for underwater diving where the gas is to be supplied from high-pressure cylinders, as in scuba, scuba replacement and surface-supplied mixed gas diving, where the component gases are blended by partial pressure addition to the storage cylinders, and the mixture storage pressure may be higher than the available pressure of the components.
(2) Helium reclaim systems, where the heliox breathing gas exhaled by a saturation diver is piped back to the surface, oxygen is added to make up the required composition, and the gas is boosted to the appropriate supply pressure, filtered, scrubbed of carbon dioxide, and returned to the gas distribution panel to be supplied to the diver again.
(3) Workshop compressed air is usually provided at a pressure suited to the majority of the applications, but some may need a higher pressure. A small booster can be effective to provide this air.
Gas booster pumps are usually piston or plunger type compressors. A single-acting, single-stage booster is the simplest configuration, and comprises a cylinder, designed to withstand the operating pressures, with a piston which is driven back and forth inside the cylinder. The cylinder head is fitted with supply and discharge ports, to which the supply and discharge hoses or pipes are connected, with a non-return valve on each, constraining flow in one direction from supply to discharge.
When the booster is inactive, and the piston is stationary, gas will flow from the inlet hose, through the inlet valve into the space between the cylinder head and the piston. If the pressure in the outlet hose is lower, it will then flow out and to whatever the outlet hose is connected to. This flow will stop when the pressure is equalized, taking valve opening pressures into account.
Once the flow has stopped, the booster is started, and as the piston withdraws along the cylinder, increasing the volume between the cylinder head and the piston crown, the pressure in the cylinder will drop, and gas will flow in from the inlet port. On the return cycle, the piston moves toward the cylinder head, decreasing the volume of the space and compressing the gas until the pressure is sufficient to overcome the pressure in the outlet line and the opening pressure of the outlet valve. At that point, the gas will flow out of the cylinder via the outlet valve and port.
There will always be some compressed gas remaining in the cylinder and cylinder head spaces at the top of the stroke. The gas in this “dead space” will expand during the next induction stroke, and only after it has dropped below the supply gas pressure, more supply gas will flow into the cylinder. The ratio of the volume of the cylinder space with the piston fully withdrawn, to the dead space, is the “compression ratio” of the booster, also termed “boost ratio” in this context. Efficiency of the booster is related to the compression ratio, and gas will only be transferred while the pressure ratio between supply and discharge gas is less than the boost ratio, and delivery rate will drop as the inlet to delivery pressure ratio increases.
Delivery rate starts at very close to swept volume when there is no pressure difference, and drops steadily until there is no effective transfer when the pressure ratio reaches the maximum boost ratio.
Compression of gas will cause a rise in temperature. The heat is mostly carried out by the compressed gas, but the booster components will also be heated by contact with the hot gas. Some boosters are cooled by water jackets or external fins to increase convectional cooling by the ambient air, but smaller models may have no special cooling facilities at all. Cooling arrangements will improve efficiency, but will cost more to manufacture.
Boosters to be used with oxygen must be made from oxygen-compatible materials, and use oxygen-compatible lubricants to avoid fire.