Using a Pump as a Compressor

Pressure exchangers are found in seawater reverse osmosis systems. A smaller pump is mechanically coupled to a larger pump, so that the energy released from the reject stream where the pressure is let down is transferred as part of the energy needed to boost the pressure in the feed stream. In that respect, they can be considered as a pump being used as a compressor, and vice versa.

In this way, the overall power consumption of the equipment per unit of product is much reduced.

Agree, except that what you describe is a pump as turbine (PAT) not a compressor. Centrifugal pumps are no use as compressors bu can work well as liquid turbines.

Reciprocating pumps will work as compressors, but are not specifically designed to work "dry" which problem must be addressed.

Depends on the pump. In most reciprocating pumps for fluid, the fluid that is being pumped cools the pump. With out the cooling effects of the fluid the pump may seize.

In addition to the basic cooling of the pump as mentioned the pump characteristics depend on the viscosity of the fluid being handled (except some pd pumps the effect is less).

In fact when we specify the pumps (say 250 lpm on ISO VG 320- say), the pumps are very often factory tested on ISO VG 32 and then the curves are translated as per VDI standards to refer to the other viscosity grades.

If you see the pump characteristics on even a screw pump (which is a pd pump) the curves are quite significantly different.

The pumps are designed to handle a specific type of fluid

The basic design, cooling mechanism, seals(glands) etc all are dependant on the fluid and hence your experiment is not likely to be an effective one.

On viceversa mostlikely you will damage rotor components which are not designed to handle the power gradient

BTW : is it a homework?

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A small engine can be easily converted to a compressor by removing the valves and reworking the head .