COOLING OF COMPRESSED AIR IN A COMPRESSOR UNIT

A. N. KABAKOV

(Presented by the scientific seminar of the Department of Mining Machines and Mining Electromechanics)

The air drawn into a compressor is always saturated with water vapor. The relationship between the moisture content and the temperature of atmospheric air is shown in Fig. 1. The presence of moisture in compressed air adversely affects the operation of pneumatic mechanisms, creates hydraulic shocks in the pipeline; at subzero temperatures, moisture, precipitating on the inner walls of the air duct, freezes, gradually narrowing the cross-section of the pipe and causing an additional pressure loss. Therefore, the degree of dryness of compressed air characterizes its quality on a par with pressure and other parameters.

Fig. 1. Moisture content in atmospheric air at a relative humidity of 80%.

For the purpose of condensing moisture, the Rules for the Design and Safe Operation of Air Piston Compressor Installations and Air Pipelines (Moscow, 1963) provide for the installation of final coolers when the temperature of the compressed air at the outlet from the last stage of the compressor exceeds 120°C.

However, the tubular-type coolers used do not provide the required cooling of the air, especially in summer. Scale

on the inner walls of the tubes sharply reduces the transfer of heat to the cooling water. Leakage in the cooler leads to humidification of the air.

In order to increase the operating efficiency of terminal coolers, at the Department of Mining Mechanics of the Tomsk Polytechnic Institute a model of a cooler with direct contact between the compressed air and the cooling water was developed and manufactured. For visual observation, the model was made of organic glass.

The installation for studying the cooler consists (Fig. 2) of compressor 1, air duct 2, valves 3, 4, 5, a measuring tank with cooling water 6, heater 7, and cooler 8.

The purpose of the study was: to investigate the effect of the velocity of the compressed air in the cooler on the temperature difference between the air and the water at the end of the cooling process; the effect of the air velocity and the level of the water–air layer on the magnitude of the air pressure drop; determination of the specific water consumption and the effect of the compressed-air pressure on the cooling process; and the effect of the temperatures of the compressed air and cooling water on heat exchange between them.

The investigations were carried out as follows. The compressor was put into operation. The compressed air, passing through the pipeline and through the heater, was heated to a temperature above 120°C and then entered the cooler, to which cooling water was supplied from the measuring tank. The amount of water was determined by weighing on scales; the time of the cooling process was recorded with a stopwatch; the velocity of the compressed air in the cooler was regulated by the flow rate with the aid of valve 3, and the thickness of the cooling-water layer in the cooler by valves 4 and 5.

The installation is equipped with measuring apparatus (Fig. 2): thermocouples and thermometers for measuring the temperature of the air and cooling water 9, differential manometers with a throttle diaphragm 10 for measuring

Fig. 2. Diagram of the installation during testing of the cooler

the flow rate of compressed air and the pressure drop in the cooler as the air passes through the water layer, mercury and spring manometers 11, and a psychrometer for measuring air humidity 12. To investigate changes in air flow rate and fluctuations of the pressure drop in the cooler, an eight-channel MPO-213 oscillograph was used.

Analysis of the results of testing the cooler with direct contact with water established that the air temperature at the end of the cooling process depends on the level of the water–air layer. This dependence is presented graphically in Fig. 3 for the case in which the velocity

Fig. 3. Dependencies: 1 — of the outlet air temperature \((t^\circ\mathrm{C})\) on the level of the water–air layer \((h)\) in the cooler at an air flow rate equal to \(1.5\ \mathrm{m}^3/\mathrm{min}\); 2 — of the pressure drop \((\Delta p)\) in the cooler on the height of the water–air layer \((h)\).

of air passage in the cooler is \(1.5\ \mathrm{m/sec}\) at an inlet temperature of \(120^\circ\mathrm{C}\). It is evident from the figure that, with an increase in the level of the water–air layer, the degree of air cooling increases; however, the pressure drop also increases.

The velocity of passage of compressed air in the cooler is limited by entrainment: with the cross section of the cooler model we studied, equal to \(0.0167\ \mathrm{m}^2\), entrainment begins at a compressed-air passage velocity above \(1.5\ \mathrm{m/sec}\).

The tests carried out on a cooler with direct contact between the air and the cooling water, using an organic-glass model, showed that the temperature difference between the air and the water at the end of the cooling process is \(2\text{–}4^\circ\mathrm{C}\) at a specific water consumption of \(3\text{–}4\ \mathrm{l}/\mathrm{m}^3\). The pressure losses in the cooler when compressed air passes through the water layer reach \(0.05\ \mathrm{at}\).

The air-cooling process in the cooler during the tests was carried out at a pressure of up to \(1\ \mathrm{ati}\) and at a constant inlet temperature of the compressed air equal to \(120^\circ\mathrm{C}\).

The influence of the temperatures of the compressed air and the cooling water on the heat exchange between them, as well as the influence of pressure on the cooling process, will be investigated further.