ON THE EFFICIENCY OF PNEUMATIC TURBINE ENGINES USED UNDER VARIABLE LOAD CONDITIONS

S. I. Shubovich

(Presented by Prof. Dr. Tech. Sci. V. T. Yurinskii)

Among some specialists in pneumatic drives for mining machines there exists the opinion that turbine pneumatic engines are not advisable for driving mining machines and mechanisms operating under variable load. Thus, for example, on this question G. I. Kusnitsyn [1] writes: “Turbine engines are favorably distinguished by their high efficiency, absence of rubbing parts, and low weight. However, these engines can be used only for driving machines operating with a constant load. Such machines include fans, pumps, etc. When these engines are used for mechanisms operating with variable load, their efficiency drops sharply.” The author of the text quoted here correctly notes that the efficiency of a turbine engine decreases when operating with variable loads; however, the conclusions he draws are not convincing, since under variable loads a decrease in efficiency is observed in all pneumatic engines. The advisability of using turbine engines under variable-load conditions can be established only by comparing their economic characteristics (efficiency or specific air consumption) as functions of load with the characteristics of pneumatic engines of other types.

In the author’s opinion, turbine engines may find application as engines for coal combines, undercutting machines, heavy impact-drilling machines, conveyors, etc. At present these machines in mines dangerous with respect to gas and dust are supplied, as is known, with gear-type engines. To determine the advisability of using turbine engines as drives for the above-mentioned machines operating with variable loads, it is necessary to compare their characteristics with those of gear-type engines.

In the present article a comparative analysis is made of the characteristics and economic indicators of two experimental turbine [2; 3] and two production gear-type [1] pneumatic engines. The main parameters of these engines are given in Table 1.

Comparison of turbine and gear-type engines when operating under underload

It can be shown [4] that, when comparing the economic indicators of pneumatic engines of different types and sizes operating under variable-load conditions, it is convenient to use curves of specific air consumption plotted as a function of the ratio of the load torque \(M_c\) to the nominal torque of the engine \(M_{\text{nom}}\), conventionally called the engine load coefficient \(K_n\).

Table 1

For comparison of the economy of the DTR-20 turbine engine with gear-type engines of the PRSh-10 and PSh-30 M types, Fig. 1 presents curves of the specific air consumptions \(q=\varphi(K_n)\) within the range of variation of the load coefficient from \(K_n=0\) (idling) to \(K_n=1.0\) (rated load of the engine). These curves were constructed graphically [4] from the external and partial characteristics \(M_e=f_1(n)\) and \(q=f(n)\), obtained

Fig. 1

during bench tests of the engines listed above [1; 5]^1). In plotting the curves \(q=\varphi(K_n)\), it was conventionally assumed that all the engines considered here are equipped with speed governors having one and the same degree of nonuniformity, \(\delta=10\%\), and that the governing characteristics of these engines are rectilinear.

From the graphs (Fig. 1) it can be seen that, in the range of load variation from \(K_n=0.25\) to \(K_n=1.0\), the turbine engine DTR-20, irrespective of the form of the load-versus-time graph, is more economical than geared engines.

If one calculates and compares the average specific air consumptions [4] of these engines for the case of a cyclic linear variation of load with time (Fig. 2), within the above-indicated limits of load variation (from \(K_n=0.25\) to \(K_n=1.0\)), it turns out that, under these conditions, the turbine engine DTR-20 is more economical than the geared engine PSh-30 M by approximately 25%^2).

Fig. 2

Here it is appropriate to note that in turbine engines, unlike geared engines, it is possible to carry out not only throttling (qualitative) regulation, but also nozzle (quantitative) regulation, which consists in the fact that, as the load changes, the speed governor brings into operation a greater or smaller number of working turbine nozzles.

In Fig. 1 a curve is plotted for the variation of the specific air consumption for the DTR-20 engine with nozzle regulation. This curve was constructed from data of speed characteristics obtained from the DTR-20 engine at a constant air pressure in the network of \(p=4\) ati and with different numbers of working nozzles. From comparison of the specific-consumption curves (Fig. 1) it can be seen that nozzle regulation provides a noticeable economic effect at low engine loads. Therefore the use of such regulation is highly expedient in a turbine engine intended to drive a mining machine operating for the predominant part of the time at low loads. The average specific air consumption of a turbine engine with nozzle regulation, under periodic linear variations of load within the limits from \(K_n=0.25\) to \(K_n=1.0\), is lower than the air consumption of a turbine engine equipped with throttling regulation by approximately 12%, and lower than that of the geared engine of the PSh-30 M type by 35%.

Thus, when operating under partial load (according to the governor characteristic), turbine engines are favorably distinguished, in comparison with geared engines, by a lower air consumption with throttling regulation, and are distinguished still more favorably when nozzle regulation is present.

^1) Tests of the DTR-20 engine were carried out by the author in the laboratory of the Tomsk Electromechanical Plant named after V. V. Vakhrushev.

^2) Obviously, with another (nonlinear) load graph, as well as with other limits of load variation, other relationships between the economic indices of these engines will occur.

Comparison of the Economic Indices of Turbine and Gear Motors Operating with Overload

Figure 3 presents curves of the specific air consumptions \(q=\varphi(K_n)\) of the motors under consideration for changes in load from the nominal value \(K_n=1.0\) to the maximum possible \(K_n=K_{n\max}\). These curves, like the curves in Fig. 1, were constructed graphically from the speed characteristics.

From the graphs (Fig. 3) it can be seen that the DTR-20 turbine motor is more economical than the PRSh-10 gear motor at any value of the load coefficient \(K_n\) permissible for this motor. In comparison with the PSh-30 M motor, the DTR-20 motor is more economical only at \(K_n=1.2\) and \(K_n>1.32\). At values of the load coefficient \(K_n=1.2 \div 1.32\), the PSh-30 M motor is somewhat more economical than the DTR-20 motor.

Fig. 3

If the mean specific air consumptions for these motors are calculated and compared under the condition of a linear change in load within the limits from \(K_n=1.0\) to \(K_n=1.25\), it turns out that the DTR-20 turbine motor is more economical than the PRSh-10 motor by 20%, and than the PSh-30 M motor by 15%. When the load varies within the limits possible for the PSh-30 M motors, from \(K_n=1.0\) to \(K_n=1.35\), the turbine motor is more economical than the PSh-30 M motor by only 8–9%.

Thus, the DTR-20 pneumatic turbine motor is more economical than gear motors not only when operating according to the governor characteristic, but also when operating according to the external characteristic—when operating with overload. However, in the latter case, as is not difficult to note, the turbine motor provides a relatively smaller economic effect in terms of air consumption than when operating under underload conditions—according to the governor characteristic.

It can be shown [4] that the economic indices of a pneumatic motor under overload conditions can be improved by increasing the starting torque and by improving the character of the curve of the variation of specific air consumption with rotational speed. In a motor of the DTR-20 type this can be achieved, for example, by using a turbine with two speed stages instead of a single-stage impulse turbine.

Figure 4 plots the curves \(q=\varphi(K_n)\) of an experimental reversible motor with a single-shaft two-stage turbine [3] and of the motors listed above\(^1\).

From these graphs it can be seen that the motor with a two-stage turbine consumes less air than any of the gear motors at any degree of overload. The average air consumption of this experimental—

\(^1\) These curves, in contrast to the curves (Fig. 3), were constructed for an operating pressure \(P=3.5\) ati, at which the experimental motor was tested.

...experimental engine, with a linear change in load from \(K_n = 1.0\) to \(K_n = 1.35\), is lower in comparison with the consumption of the PSh-30 M engine by 13%, and in comparison with the DTR-20 turbine engine by 9–10%. Thus, the use of a turbine with two speed stages in a pneumatic engine can noticeably improve the economy of the latter under overload conditions.

Fig. 4

Conclusion

From the above analysis of the operation of turbine and gear-type engines it follows that, under variable-load conditions, turbine engines are more economical than gear-type engines. Consequently, the field of application of turbine engines is not limited only to driving machines operating with constant load, as is erroneously stated in the literature [1].

The use of turbine engines to drive mining machines operating with variable loads can provide a substantial saving of costly compressed air.

References

1. Kusnitsyn G. I. Pneumatic Engines of Mining Machines and Mechanisms. Ugletekhizdat, 1953.

2. Shubovich S. I. Experimental study of friction and ventilation losses in a turbine stage. Izvestiya TPI, vol. 75, 1954.

3. Shubovich S. I., Lvov V. I., Sharapov R. D. Results of tests of an experimental pneumatic reversible two-stage turbine. Izv. TPI, vol. 85, Tomsk, 1957.

4. Shubovich S. I. Load characteristics for evaluating the economy of operation of pneumatic engines. Izv. TPI, 85, Tomsk, 1957.

5. Balykov V. M. The KKP-1 Coal Combine for Steeply Dipping Seams. Ugletekhizdat, 1952.