Numerical Investigation Of Heat Transfer On Screw Compressor Rotors.

Abstract

Since dry screw compressor rotors and housing are subjected to high
temperatures which cause their deformation, they either may not operate at high pressure
ratio, or they must be cooled. However, the gas temperature is high only in the high pressure
region of the screw compressor therefore the domain where the heat is transferred to the
rotors is limited to only a small portion of the rotors and housing. Also, since the rotors
revolve at high speeds while residing only briefly within areas of different gas temperatures
during one cycle, the rotor temperature becomes virtually uniform across any cross sectional
area perpendicular to the rotor axis and its value is somewhere between the highest and
lowest gas temperature at that cross section area. At the same time, due to the relatively high
thermal conductivity of the compressor components, which are made of metal, heat transfer
by conduction is considered substantial and the rotor body temperature attained is a result of
a balance between the heat received from the gas and its dissipation in regions of lower gas
temperature. Consequently, it appears that rotor cooling is required only in the compressor
high temperature region to keep rotor temperatures at a reasonable level.
Therefore, an effective means of cooling the rotors could be to inject a small quantity of
flashing liquid, preferably water into the casing, at the high pressure port end in any
circumferential position. The liquid would then impinge on the rotors at a rate such that it
would be instantly evaporated by contact with them. To confirm these principles and quantify
the heat transfer rate and required flow rate of liquid injected for rotor cooling, a complex
numerical investigation by using a CFD code has been performed. The results indicate that
dry compressors may run at pressure ratio up to 8:1. This is substantially higher than the
normal pressure ratios attainable in such machines. It is also shown that due to the high
rotational speed and intermittent passing through hot and cold gas areas, the rotor
temperature is far lower than that of the gas. Since the heat transfer rate between the gas and
the rotors is very low, it is possible to cool the rotors by only relatively small quantities of
flashing fluid, such as water. The experimental investigation was performed to confirm these
estimations. The result of this investigation is that only minor modification is required for oil
free compressors to be made to operate with much higher pressure ratios in a single stage.