IMPROVING THE EFFICIENCY OF COOLING COMPRESSED AIR IN COMPRESSORS

V. F. Kutsepalenko, A. N. Kabakov, B. A. Tikhonov

(Submitted by the scientific seminar of the Department of Labor Protection)

Compressed air, as a source of energy, is widely used in industry. For the production of pneumatic energy in the USSR, as of 1964, about 250 thousand compressors with a total drive power of about 13 million kW were in operation.

In the field of pneumatics, the problem of increasing the efficiency of the processes of producing compressed air in compressors remains urgent to the present day. One way of solving this problem is to improve the method of cooling compressed air.

In compressor installations, air cooling (intermediate and final) is carried out, as a rule, by shell-and-tube apparatuses (Fig. 1, a). The named apparatuses have a number of irremovable disad—

Fig. 1. Structural diagrams of apparatuses for cooling compressed air in compressors

residues, the most important of which are the following: formation of deposits on the working surfaces of apparatuses, insufficient cooling of the compressed air and an increased content in it of moisture and oil vapors. These shortcomings adversely affect quality and increase the danger of using pneumatic energy (the possibility of hydraulic shocks and internal freezing in pneumatic networks, the possibility of explosions and fires).

As a result of searches for an improved method of cooling compressed air, we developed a bubbling-and-spray type apparatus [1, 2]. In this apparatus (Fig. 1,b), cooling is carried out on a new principle—by direct contact of the hot compressed air with the cooling water.

A specimen of the apparatus manufactured in the departmental workshop was studied in the laboratory and, in 1968, also under semi-industrial conditions (at the Tomsk Electromechanical Plant). The aim of the study was to investigate the heat-exchange process and to determine the principal characteristics of the cooling apparatus—the heat-transfer coefficient, the degree of cooling, the hydraulic resistance to the passage of compressed air, the consumption of cooling water, and other indices.

The cooling process in the named apparatus, as in any heat exchanger, depends on a number of variable physical quantities characterizing the interacting components (air—water) and the parameters of the apparatus (effective cross section, height of the water–air layer). Dimensional analysis of these quantities made it possible to obtain a calculated criterion equation for the heat-transfer coefficient, namely:

\[ \frac{K d_{\mathrm{e}}}{\lambda} = A \left( \frac{\omega d_{\mathrm{e}}}{\nu} \right)^{B} \cdot \left( \frac{h_{v-b}}{d_{\mathrm{e}}} \right)^{C}. \tag{1} \]

After experimental determination of the coefficient \(A\) and the exponents \(B\) and \(C\), equation (1) is reduced to the form

\[ K = 2.6 \frac{\lambda \omega^{0.93} h_{v-b}^{0.07}}{d_{\mathrm{e}}^{0.14} \nu^{0.93}}, \tag{2} \]

where \(K\) is the heat-transfer coefficient \((\mathrm{W}/\mathrm{m}^{2}\,\mathrm{deg})\);

\(\lambda\) is the thermal-conductivity coefficient of air \((\mathrm{W}/\mathrm{m}\,\mathrm{deg})\);

\(\omega\) is the velocity of air motion in the apparatus \((\mathrm{m}/\mathrm{s})\);

\(h_{v-b}\) is the height of the water–air layer in the apparatus \((\mathrm{m})\);

\(d_{\mathrm{e}}\) is the equivalent diameter of the useful cross section of the apparatus \((\mathrm{m})\);

\(\nu\) is the coefficient of kinematic viscosity \((\mathrm{m}^{2}/\mathrm{s})\).

Equation (2) expresses the similarity conditions for cooling apparatuses and makes it possible to construct a parametric series for them.

The heat-transfer coefficient was also calculated by the Newton–Richmann formula

\[ Q = KS \Delta t_{\mathrm{av}} \tau, \tag{3} \]

where \(Q\) is the amount of heat removed by water from the air \((\mathrm{W}\cdot\mathrm{h})\);

\(S\) is the effective cross section of the apparatus \((\mathrm{m}^{2})\);

\(\Delta t_{\mathrm{av}}\) is the average temperature difference between the air and the water \((^{\circ}\mathrm{C})\);

\(\tau\) is the duration of heat exchange \((\mathrm{h})\).

In formula (3) all quantities were determined experimentally. Comparison of the values of the coefficient \(K\) obtained from expressions (2) and (3) showed that the maximum deviation of the experimental data from the calculated data does not exceed 10%.

The operation of the bubbling–spray type apparatus was investigated in the form of several series of experiments; for each series, the parameters of the compressed air and cooling water were set and maintained constant (velocity, pressure, temperature and humidity of the air, mass flow rate of the water and its inlet temperature), as was the height of the water–air layer in the apparatus. The necessary quantities were measured 8–10 min after the start of the experiments, during which time the heat-exchange regime in the apparatus became stabilized.

After processing the experimental data, the graphical dependences shown in Figs. 2, 3, and 4 were constructed. The table below gives some characteristics of the tested specimen of the apparatus, as well as results obtained by calculations on the basis of the heat-balance equation and using the experimental data.

Table

In experiments carried out on the compressor installation of the Tomsk Electromechanical Plant, the apparatus was used as an intermediate cooler for a 160V-20/8 compressor under the following initial conditions:

After cooling in the apparatus, the air temperature decreased to 6–12°C, and the water temperature was 5–7°C. At the same time, the temperature difference between the water and the air at the outlet from the apparatus was within 1–5°C, whereas in the shell-and-tube cooler with which the indicated compressor was equipped, it was 20–25°C.

The depth of air cooling in the apparatus can be estimated by the following relation:

\[ \eta_0 = \frac{t_1 - t_2}{t_1}, \tag{4} \]

where \(t_1, t_2\) are the air temperatures at the inlet to and outlet from the apparatus.

The quantity \(\eta_0\), in physical terms, represents the degree of air cooling and lies within the limits \(0 \le \eta_0 \le 1\). The extreme values of the degree correspond to the cases of a complete absence of cooling (\(t_2 = t_1\)) or complete cooling (\(t_2 = 0\)).

The degree of cooling is an important characteristic of a heat-exchange apparatus. It reflects the level of its perfection both with respect to the cooling method and to the design.

Using the experimental results, we find that in an apparatus of the bubbling-spray type the degree of cooling is about 0.8.

The difference \(1 - \eta_0\) will determine the magnitude of the undercooling of the air. This quantity can also be characterized by the difference between the outlet temperatures of the air and the water. The ideal case will correspond to equality of these temperatures, i.e., when the thermal potentials of the contacting media are equalized.

The magnitude of the undercooling, and consequently also the degree of cooling, depend mainly on the air velocity and the height of the water-air layer. As can be seen from Fig. 2, at \(h_{в-в} = 400\) mm the magnitude

Fig. 2. Dependence of the difference between the temperatures of the air and water at the outlet from the apparatus on the height of the water-air layer.

of the undercooling for this design of apparatus will be the same for any air velocity.

Considering the graph in Fig. 3, we find that, according to the conditions of hydraulic resistance, a water-air layer height of 400 mm represents its upper permissible limit. Beyond this limit, the operating region of the apparatus is not rational, since it is impossible to carry out the motion of the air at a sufficient velocity while maintaining acceptable values of hydraulic resistance. And although increasing the velocity causes a favorable change in the heat-transfer coefficient (Fig. 4), the upper value of the velocity should nevertheless be adopted with allowance for its influence on the magnitude of the hydraulic resistance of the apparatus and on entrainment during bubbling of the air in it.

Fig. 3. Dependence of compressed-air pressure losses on its velocity in the apparatus

Fig. 4. Effect of the air velocity in the apparatus on the heat-transfer coefficient from air to water

In conclusion, we note that cooling compressed air on the principle of direct contact largely eliminates the disadvantages inherent in shell-and-tube coolers and has other advantages over the latter, namely: a heat-transfer coefficient several times higher, almost half the hydraulic resistance, a greater (by 15–20%) degree of air cooling, and a simpler design.

Only in terms of the specific consumption of cooling water ($7\ \text{l}/\text{m}^3$) does the proposed apparatus fall short of the shell-and-tube type, for which this consumption is $4\ \text{l}/\text{m}^3$.

For the effective use of bubbling-and-spray apparatuses in compressor installations, it is additionally necessary to resolve certain questions, such as automation of regulation of the height of the water–air layer, improvement of the apparatus design, etc.

REFERENCES

1. A. N. Kabakov. Cooling of compressed air in compressor installations. Publishing House of TPI, vol. 162, 1967.

2. Official Bulletin of the Committee for Inventions and Discoveries under the Council of Ministers of the USSR, No. 36, p. 31, “Heat Exchanger,” 1968.