ISSN 00406015, Thermal Engineering, 2015, Vol. 62, No. 12, pp. 906–910. © Pleiades Publishing, Inc., 2015. Original Russian Text © V.M. Batenin, Yu.A. Zeigarnik, A.S. Kosoi, V.V. Datsenko, M.V. Sinkevich, 2015, published in Teploenergetika.

STEAM TURBINES, GAS TURBINES, HYBRID SYSTEMS, AND AUXILIARY EQUIPMENT

Experimentally Studying of Axial Compressor Operation with Steam V. M. Batenin, Yu. A. Zeigarnik, A. S. Kosoi, V. V. Datsenko, and M. V. Sinkevich Joint Institute of High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 13/3, Moscow, 125412 Russia email: [email protected] Abstract—Steam offers numerous benefits when used as the working fluid in thermodynamic cycles. In low temperature cycles, where thermal energy is switched from one temperature potential to another (thermal transformers), steam is much less commonly used as a working fluid than in hightemperature cycles. The deficiencies of difficulties in using steam in thermal transformers include low pressure at the working temper atures and hence large specific volume. A compressor capable of high productivity having high discharge and relatively large increase in pressure is required. To that end, a multistage axial compressor from an airplane aircraft engine may be employed. To confirm the viability of this approach, the compressor of an AL21 air plane aircraft engine is tested on a custom test bench. Experimental results are presented for a multistage axial compressor working with steam, when the input pressure is 0.5–5 kPa. Keywords: thermal transformer, heat pump, refrigerator, steam, steam compressor, test results, compressor characteristics DOI: 10.1134/S0040601515120022

Steamcompression thermodynamic cycles are widely used for conversion of the temperature poten tial of energy. Such systems may be characterized as are called thermal transformers. Most often, however, they are called either refrigerators or heat pumps, depending on their function. In Fig. 1, we show a basic thermal transformer. Various working fluids may be used for these sys tems (for example, refrigerants, butane, propane, or ammonia). Each has its own advantages and disadvan tages. Water (steam) is also used as the working fluid for the steamcompression cycle. Steam has the fol lowing basic advantages. 1. Minimal environmental impact, safe and simple equipment, availability, and low cost. 2. Thermophysical properties ensuring intense heat transfer.

2

3

5

1

6

4

8 7 Fig. 1. Basic structure of thermal transformer: (1) cold heat source; (2) evaporator (of contact type); (3) lowpressure steam; (4) compressor; (5) condenser; (6) heat consumer; (7) condensate; (8) compressor drive.

3. High stability: no change in properties with time or temperature variation. 4. The possibility of using the same working fluid in the thermodynamic cycle and in the technological process that consumes thermal energy. The same water may operate in the thermodynamic cycle and in the heating or airconditioning system. Disadvantages of steam in thermal transformers include large specific volume at the working pressures and relatively high kinematic viscosity. These proper ties complicate the selection of the compressor for the thermal transformer. A compressor capable of high discharge and relatively large increase in pressure is required. The best available compressors are those of large gas turbines. In the development of a higheffi ciency combinedcycle system, researchers at the Joint Institute of High Temperatures of the Russian Academy of Sciences and MMPP Salyut used a steambased compression heat pump with the com pressor of an AL21 aircraft engine in the cogenera tion part system [1]. To confirm that efficient steambased thermal transformers may employ an air compressor, we mod ernized the AL21 compressor and tested it with steam over a wide range of operating conditions. Experi ments were conducted so as to confirm that the air compressor may operate with steam and its character istics were determined with a steam pressure of 0.5– 5 kPa at the compressor input. The AL21 compressor has numerous controllable guide vanes: two autonomously controllable groups of five vanes. We needed to find optimal blade inclina

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EXPERIMENTALLY STUDYING OF AXIAL COMPRESSOR OPERATION WITH STEAM 1

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Fig. 2. AL21 compressor in sealed housing: (1) guide control mechanisms; (2) AL21 compressor; (3) electric motor; (4) adjust able choke; (5) sealed pathways; (6) sealed casing.

tions and to develop a program for controlling the guides. Modernization of the AL21 compressor included the following basic operations: ⎯design of a sealed housing for the compressor; ⎯separate connection of the rotary mechanisms of the guide vanes to the controlling step motors; ⎯replacement of roller bearings by slip bearings for the rotor; ⎯replacement of the gasturbine drive by an elec tric motor; ⎯introduction of a coil with a controllable choke in the output diffusor. The complexity in implementing these operations is associated with the experimental conditions—in particular, the extremely low pressure (up to 0.5 kPa). Therefore, it was essential to ensure bearing opera tion with water lubricant; cooling of the rotor of the electric motor; and prevention of air intake through gaps in the flange joints and at the numerous techno logical and measuring connectors. To reduce the probability of air leakage into the AL21 compressor, all the moving parts of the mech anisms that require sealing (the control mechanisms of the guides, the bearings, the compressor drive) are placed within a sealed casing (Fig. 2). All the supply lines (water delivery and discharge, pneumatic and electrical lines for the measuring signals, the power supply) pass through the casing in special sealed pathways. THERMAL ENGINEERING

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The use of slip bearings with water lubrication to replace roller bearings with oil lubrication should rule out oil leakage into the compressor channel. The leak age of water into the compressor channel is permissi ble. Accordingly, the sealing of the waterlubricated bearings is much simplified. However, difficulties may arise with the lubricant itself, since water in the lubri cation system may be overheated by tens of degrees. Since the compressor drive must be placed within a sealed casing at low pressures, the gas turbine usually employed must be replaced. Design considerations exclude the use of familiar electric motors at the high rotor speeds required in the compressor. Accordingly, a special highspeed electric motor was designed for the AL21 compressor [2]. The required power at the compressor drive is proportional to the pressure at the compressor input. Therefore, a 350kW motor was designed for compressor operation in the given pres sure range. To ensure control of the compressor’s rotor speed, a frequency converter was connected to the drive. The electric motor is integrated into the com pressor; its rotor is combined with the compressor shaft, without special bearings. The electric motor is cooled by water. The rated rotor speed for the motion in lowpressure steam is 9000 rpm. For the tests, we constructed a special test bench with a sealed loop consisting of a compressor in a sealed housing; a tubeintube heat exchanger; a pipeline con necting the compressor output to the heatexchanger input; and a return pipe connecting the compressor input to the heatexchanger output (Fig. 3). A vacuum pump ensures the required pressure in the loop. The

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2

1 6000

700

Diam. 1135

Coolingwater output

4

Diam. 800

3600

Coolingwater input

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Fig. 3. Test bench for compressor: (1) heat exchanger; (2) compressor in sealed housing; (3) vacuum pump; (4) choke.

compression of the working fluid (steam) may be adjusted by means of the choke at the output from the compressor. The steam, which is heated on compres sion, is sent to the heat exchanger, where it is cooled by water to the specified temperature and sent to the com pressor input. The temperature of the working fluid (steam) is measured at the compressor input, after each stage, and at the compressor output. The pressure is mea sured at the compressor input, after the zero, first, third, and seventh stages, and at the compressor out put. The steam flow rate is measured at the compressor input. Two vibration sensors are installed at each bear ing, making measurements in the horizontal and ver tical planes, respectively. The rotor speed is measured in three independent channels. All the signals are recorded by a computer with a 2Hz response fre quency. For the sake of clarity, some of the pressure measurements are sent in parallel with the computer to oil piezometers. The tests were conducted by the following method. Preliminary evacuation reduced the absolute pressure in the test loop to 3–4 kPa. Since small quantities of liq uid water are always present in the loop, the preliminary pressure corresponded to equilibrium at the ambient temperature. If the readings of the wet thermometer and

the measured corresponded to the familiar equilibrium relationship, we concluded that the steam in the loop is of acceptable purity. Then cooling water was supplied to the heat exchanger and the measuring system was tested. After confirming that the test bench is ready for use, the electric motor was turned on, the adjustable choke was set to the completely open position, the rotor accelerated to the specified speed, and the guides were installed according to the test specifications. After the rotor reached the specified speed (point 1 in Fig. 4), a pause of 5–6 min permited stabilization. Then the mea surements were made. (From the complete set of data, a sample from a small time interval was selected for sub sequent analysis.) If the measurement was acceptable and the compressor was in the stable operating zone, the choke is moved to the next position. This cycle was repeated for each choke position (points 2–5 in Fig. 4) until the compressor left the stable operating zone. At that point, the speed was reduced to 60% (5400 rpm), the choke was completely opened, and the guides were adjusted so that the compressor returned to the stable operating zone. Then the speed was increased in accor dance with the test specifications and all the operations were repeated until the compressor again left the stable operating zone. For each tested speed, the pressure was repeatedly increased. Each time, the control program THERMAL ENGINEERING

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EXPERIMENTALLY STUDYING OF AXIAL COMPRESSOR OPERATION WITH STEAM πc

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πc 10 8

m Li

o it

ta fs

e bl

ti ra e op

on

i L im

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to

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le t ab

e op

ion r at

n = 100% 100%

90% 90%

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30%

n = 80%

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80%

40% 3

60 50%

Speed isoline

100% choke opening

70

90 Gre, %

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ηco

Fig. 4. Sequence of test conditions for a compressor at a fixed speed: πc, degree of pressure increase; Gre, reduced flow rate of working fluid.

for the guides was corrected. Of the many possible con trol programs, the version that ensured the best com pressor characteristics was selected. Such optimization of the guide control program was repeated at several speeds. To find the optimal guide control program when working with steam, the control program when working with air was taken into account, threedimensional cal culations were conducted using special software, and experiment design is undertaken on the basis of the available test results [3]. The tests showed that, once the compressor had begun unstable operation, it could not easily be returned to the stable operating zone. At speeds above 60% of the rated value, the compressor cannot be returned to the stable operating zone by adjustment of the guides and complete opening of the choke. The compressor’s stable operating zone depends greatly on the control program for the guides. The first experiments, based on the control program developed for the guides when using air, proved to be unsuccess ful. Therefore, we have developed a special optimal guide control program taking account not only of the speed but also of the pressure increase. In the tests, we set the following minimum require ments: increase in the pressure by a factor of at least 7.5; compressor efficiency no less than 0.8; steam flow rates of 210000 m3/h. The results showed that these require ments may be met. In Fig. 5, we show the experimental characteristic of the compressor. It is plotted on the basis of the fol No. 12

η

lowing formulas for the reduced flow rate Gre and reduced speed nre

G re = Gac

pre Tm G re.r ; pm Tre

Tm n re.r , Tre where Gac is the actual mass flow rate measured at the compressor input, kg/s; pre, Tre are the reference pres sure and temperature, respectively; pm is the measured absolute pressure at the compressor input, kPa; Tm is the measured temperature at the compressor input, K; Gre.r is the rated reduced volumetric flow rate (the reduced flow rate, corresponding to 210000 m3/h at a pressure of 0.5 kPa and a temperature of 273 K at the compressor input); nac is the actual measured speed, rpm; nre.r is the rated speed, rpm. Note that, once the minimal requirements had been met, the financing for the tests was discontinued. Calcu lations indicate great scope for improving the compres sor characteristics. Therefore, it would be expedient to continue the research. However, even the results already obtained demonstrate the possibility of using the AL21 aircraft compressor in thermal transformers. Such sys tems may boost the temperature by 30–40 K and, pro spectively, 50–60 K according to our calculations. In Fig. 6, we show the dependence of the actual (continuous curve) and theoretically attainable (dashed curve) conversion factor of the thermal transformer based on the AL21 compressor on the temperature rise. The efficiency of the thermal transformer based on the AL21 compressor when using steam as the working fluid is relatively close to what is theoretically attainable. That indicates the great commercial potential of such systems. The AL21 compressor has been tested with input pressures up to 0.5 kPa, corresponding to cold source temperature of 273 K. In an aircraft engine, this compressor operates at considerably higher pressures (output pressures up to 1.5 MPa). At such pressures, nre = nac

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Fig. 5. Experimental characteristic of the AL21 compres sor when working with steam: ηc, compressor efficiency.

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(2) To ensure the required performance, the guide control program when working with steam must be significantly different from that used when working with air. (3) It is expedient to continue tests of the AL21 compressor so as to further optimize the guide control program. That would expand the applicability of the compressor and somewhat increase its efficiency over the whole operational range.

Conversion factor, kW(th)/kW(el)

35 30 25 20 15 10

REFERENCES

5 0 10

20

30 40 Temperature rise, K

50

60

Fig. 6. Dependence of the conversion factor on the tem perature rise for a thermal transformer based on the AL21 compressor.

working fluids at temperatures up to 470 K may theoret ically be used in the thermal transformer, but the prac tical expediency of such high temperatures is dubious. CONCLUSIONS (1) Tests of the AL21 compressor with steam at input pressures of 0.5–5 kPa indicate that this com pressor may be used in thermal transformers.

1. O. N. Favorskii, V. M. Batenin, Yu. A. Zeigarnik, V. M. Maslennikov, A. N. Remizov, I. T. Goryunov, A. K. Makhan’kov, V. Yu. Vasyutinskii, S. I. Pishchikov, Yu. N. Sokolov, Yu. S. Eliseev, V. E. Belyaev, A. S. Kosoi, and M. V. Sinkevich, “The PGU MES60 combinedcycle (steam–gas) installation with steam injection and a heat pump for the Mosenergo power system,” Therm. Eng. 48, 751–760 (2001). 2. V. E. Belyaev, V. N. Beschastnykh, A. S. Kosoi, and A. G. Chemiya, RF Patent No. 2450218, Otkrytiya. Izobreteniya, No. 13 (2012), (registr. 16.07.2010, publ. 10.05.2012). 3. S. O. Sereda, F. Sh. Gel’medov, and N. G. Sachkova, “Calculated assessments of the changing characteristics of a multistaged axial compressor due to the effect of water evaporation in its flow path,” Therm. Eng. 51, 924–930 (2004).

Translated by Bernard Gilbert

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