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Experimental investigation of a machine tool cooler using hot-gas by-pass valves for temperature control.

INTRODUCTION

The main spindle of a machine tool is warmed up during high-speed machining and the spindle deviates from the center of the column, which results in poor accuracy consequently. The machine coolers are essential accordingly to provide the best solution of coolant (oil or water) temperature in avoiding the deviation of spindle centerline in high accuracy machining. However the cooling load may vary over a wide range due to occupancy, machining loading or unloading and ambient weather variations. Some control schemes for system cooling capacity control are vital for the accuracy of coolant temperature. Capacity modulation with hot-gas by-pass scheme can reduce compressor cycling, decrease the starting load, provide good oil return, and avoid temperature fluctuations if properly designed. Therefore, the machine coolers with proper capacity control scheme and better durability of solenoid valves are desirable for high precision machining.

Hot-gas by-pass approach is widely accepted for capacity control in small refrigeration system. Different types of hot-gas by-pass methods as well as the fundamental of application were demonstrated in ASHRAE Refrigeration Handbook (ASHRAE, 2006). Yaqub et al. (2000) investigated the capacity control of a vapor compression refrigeration system by injecting hot-gas refrigerant into the suction line of the compressor to regulate the loading of the evaporator. A comparative study was also presented through different by-pass schemes in terms of the system coefficient of performance and operating temperatures. Besides, Cho et al. (2005) presented the performance of showcase refrigeration system during on-off cycling and hot-gas defrosting. It also revealed that the hot-gas by-pass method presented higher refrigeration capacity and less temperature fluctuation than on-off cycling. Furthermore, a comparison of hot-gas by-pass and suction modulation for capacity control in the refrigerated shipping container were investigated by Tso et al. (2001). The results displayed that both methods proved to be energy-efficient especially under lower evaporator load condition. Byund et al. (2008) investigated the feasibility and performance of the hot-gas by-pass method to retard the formation of frost in a heat pump system. Experimental results showed that the hot-gas by-pass method is useful not only for retarding the formation of frost but also for improving COP and capacity. Besides, the hot-gas by-pass defrosting method was investigated through an air-to-water heat pump by Huang et al. (2009). The results showed that hot-gas by-pass scheme could provide smaller indoor temperature fluctuation and deserve further investigation.

The cooling process of the machine tool cooler is a rather nonlinear and time-varying dynamic behavior. Although many existing control schemes such as inverter driven and PID control have been proposed for temperature control, there still exists the cost-effective concern for controller development in industrial application. The pulse width modulation (PWM) controller which produces a serious of discontinuous pulses with a fixed amplitude and variable width, has become popular in industrial application for ON-OFF control power converter and stepper motors (Lu et al., 2010). Besides, Taghizadeh et al. (2009) investigated the modeling and identification of a solenoid valve for PWM control applications. A PWM-driven pneumatic fast switching valve was presented as well. Furthermore, Shih et al. (1998) demonstrated the application of fuzzy control and modified PWM control method to control the position of a pneumatic cylinder. The experiment results showed that system has the advantage of both good performance and low cost with the proposed PWM controller. Some PWM ICs (such as TL 494) are commercial available with cost-effective consideration in the application of PWM controller.

Selection and sizing for cost-effective throttling device is essential for control of refrigeration system. A capillary tube has been widely used as a throttling device in small refrigeration and air-conditioning systems due to the advantages of simplicity, low cost, and low starting torque for a compressor. An empirical correlation and rating charts for the performance of adiabatic capillary tubes with alternative refrigerants has been developed by Chio et al. (2004). Furthermore Ding et al. (2005) presented the performance improvement of the air-to-water chiller by using an auxiliary capillary tube parallel to thermostatic expansion valve (TEV) in the application of a wide ambient temperature range. Besides, the study of using TEV to regulate the refrigerating system to operate efficiently was conducted by Yu et al. (2006).The strategy of improving the efficiency of heat pump system by adding a small amount of heat on TEV has been investigated by Gao (2010). The results showed that the strategy of TEV heating was both technologically practical and cost-effective. Besides, Mithraratne et al. (2000) developed a numerical model to simulate the transient behavior of a water cooling evaporator controlled by a TEV. The results demonstrated the characteristic of TEV and the nature of input disturbance. Furthermore, Ibrahim (2001) investigated the effect of sudden changes in evaporator on the performance of a dry-expansion refrigeration system controlled by a TEV. The analyses displayed that sudden changes might result in unstable system for a period of time even under stable operation.

In this study, the experimental investigation using hot-gas by-pass scheme for capacity control were conducted to evaluate the performance of temperature control for a machine tool cooler. To reduce the ON-OFF frequency of solenoid valve for by-pass control, different time delays were experimented under various loading condition. The temperature control accuracy of the machine tool under different delay duration and loading will be presented extensively. Effects of using capillary tube and thermostatic expansion valve along with different capacity control scheme will be investigated as well. Power consumption of the cooling system will be measured and analyzed by comparison with throttling devices under specific loading and delay time control.

EXPERIMENTAL DEVICES AND TESTING

System Description

The schematic layout of the experimental set up for a machine tool cooler system is shown in Figure 1. Experimental facility was arranged and set up for testing hot-gas by-pass control scheme and comparison of two throttling devices, i.e. capillary tube and thermostatic expansion valve (TEV) respectively. Typically, a machine tool cooler system consists of compressor, air-cooled finned-type condenser, throttling device and shell-and-coil (evaporator) cooler. Secondary chilled coolant (oil or water) was pumped into the machine tool cooling system to cool down the heat generation during machining through a coolant (water) storage tank. The upper part of experimental device was composed of four main components (i.e. compressor, condenser, throttling device and evaporator) of typical vapor compression refrigeration system. Environment-friendly refrigerant (HFC-407C) was used as the working fluid. Two throttling devices including a capillary tube and a TEV were installed in parallel to compare the accuracy of temperature control for the experimental system. Stop valves, temperature sensors pressure gauges and sight glasses were assembled at both inlet and outlet of all main components for easy replacement and measurement. Furthermore, the lower part of the experimental rig is the coolant loop to produce chilled water for cooling down the heat generation during high-speed machining. It consists of the coolant tank, pump, water flow meter, cooler (evaporator) and silicon controlled rectifier (SCR) load simulator. The SCR load simulator modulated heater was used to generate the precise heating load for machine tool spindle ranging from 0 to 1 kW (1.34 hp). A regulation valve along with a flow meter was also installed to regulate the steady flow rate of coolant to match the flow rate of commercial available machine tool cooler. The coolant tank, heater and all of the piping loops were well insulated, so that the heat losses might be neglected.

[FIGURE 1 OMITTED]

Two high quality RTD (PT-100[OHM]) sensors with an uncertainty of 0.1 [degrees]C (0.18 [.sup.0.F]) were used at the inlet and outlet of evaporator. All of the experimental data were recorded at steady state. The variation of temperature was recorded simultaneously by a multi-channel data logger (HIOKI, MODEL 8421-51) with an uncertainly of 0.05% of reading. The power consumption of compressor was measured on-site by a power meter (HIOKI, model 3169-20) with an uncertainty of 0.01% of full scale. Performance analyses of temperature control of the cooler system could be measured and compared under different heating load and coolant temperature. All of the experiments were performed in the environmental testing room to ensure the least influence by the ambient condition. The testing room reaches the design specification of temperature ranging from 0 [degrees]C to 45 [degrees]C (32 [.sup.0.F] to 113 [.sup.0.F]) with an uncertainty of 0.5 [degrees]C (0.9 [.sup.0.F]) controlled by PID controller, and relative humidity ranging from 30% - 90% RH with an uncertainty of 3%.

Hot-gas By-pass Control Loop

The hot-gas by-pass loop consists of a solenoid valve which was actuated by a pulse width modulation (PWM) controller. It could provide extra load (high temperature hot gas) from discharge line of compressor to compensate the decrease of evaporator load so that the coolant temperature can keep fixed steadily. If the evaporator load decreased, the solenoid valve would be opened to provide additional hot-gas to maintain constant coolant temperature, and vice versa. The schematic block diagram of PWM controller is displayed in Figure 2. The PWM temperature controller includes a PT-100 temperature sensor and transducer, a signal comparative control circuit, a commercial available PWM IC (TL-494) and a solid state relay (SSR) set PT-100 temperature sensor is coupled to the outlet of chilled water of machine tool cooler. A comparator was used to compare the temperature of the chilled water with a reference and further to obtain an offset value. The comparative control relay circuit receives and processes the offset value and sends a digital signal depends on the offset value. The PWM control circuit which includes solenoid and solid state relay (SSR) generated a duty ratio (the ratio of duration for ON and OFF operation) signal to drive the hot-gas by-pass solenoid to modulate the temperature of the cooling water of the machine tool cooler.

[FIGURE 2 OMITTED]

Aside from temperature control accuracy of the cooling water, the durability of the solenoid valve for by-pass control is another concern. It is essential to reduce the frequency of ON-OFF operation for by-pass solenoid valve without sacrificing the accuracy of temperature control. By adjusting the duty cycle ratio of PWM IC (adjusting the resistance and capacitance), different output pulse with the same amplitude and variable width could be obtained. Therefore, the operation of by-pass solenoid valve might be controlled precisely with different duration. Delay time of ON-OFF control for the by-pass valve could be achieved accordingly. In this study, the heating load of 200W (0.27 hp, low speed machining) and 600W (0.80 hp, high speed machining) were chosen for experimental investigation. The delay time of by-pass solenoid time controlled by PWM controlled was investigated to examine the compromise of ON-OFF frequency and temperature control accuracy. Furthermore, different throttling device including capillary tube and TEV were conducted to examine the variation of power consumption of the cooler system under different loading and delay time of solenoid valve. Moreover, the control algorithm as well as PID controller for suction valve regulation was the same as that of hot-gas by-pass scheme.

RESULTS AND DISCUSSION

The temperature control accuracy as well as the durability of hot-gas by-pass solenoid valves was investigated through the field measurement of the experimental rig. The experiments of delay time at 5 seconds through PWM controller for bypass solenoid valve were conducted under low machining loading. A capillary tube was used as a throttling device. The set point of temperature at the outlet cooling water was set at 20 [degrees]C (68 [.sup.0.F]) with control accuracy of 0.2 [degrees]C (0.36 [.sup.0.F]) through the hot-gas by-pass valve. Figure 3 (a) depicts the variation for water temperature of the machine tool cooler recorded from data logger for 15minutes (900 seconds). As the temperature of cooling water decreased to the controlled limit at 19.8 [degrees]C (67.6 [.sup.0.F]), the delay time of 5 seconds controlled by PWM technique kept the by--pass solenoid valve opened for extra 5 seconds. Then the by-pass valve was closed, the temperature of cooling water increased accordingly until it reached the upper control limit at 20.2 [degrees]C(68.4 [.sup.0.F]). The same control algorithm for time delay of 5 seconds was manipulated to extend the opening duration. The experimental results revealed that the accuracy of temperature control at 0.2 [degrees]C(0.36 [.sup.0.F]) could be achieved through the extension of opening duration of 5 seconds with the ON-OFF frequency of 10 times. It also presented the strategy of delay time by PWM was feasible in reducing the ON-OFF frequency without sacrificing the control accuracy of 0.2 [degrees]C (0.36 [.sup.0.F]).

[FIGURE 3 OMITTED]

Another test was conducted to examine the effects of the increase of delay time at 15 seconds under the same loading condition. As it was shown in Figure 3 (b), the control accuracy of 0.2 [degrees]C (0.36 [.sup.0.F]) could not be obtained because the delay time for valve opening has been increased. Even though the ON-OFF frequency could be reduced to 8 times, the extra heat generation from hot gas or the additional cooling capacity would cause poorer accuracy for temperature control. However, if the requirement of control accuracy for cooling water was not so serve, delay time of 15 seconds could provide satisfactory of 0.3 [degrees]C (0.54 [.sup.0.F]) with reduction of ON-OFF times (10 times for 5 seconds delay and 8 times for 15 seconds delay) of bypass solenoid valve. It also revealed the durability of solenoid valve could be improved while the tolerance of temperature control accuracy was allowable.

Under the condition of higher machining load at 600 W (0.80 hp), experiments have been investigated through the similar control algorithm with delay time at 5 seconds and 15 seconds respectively. As it was displayed in Figure 4 (a), the control accuracy at 0.2 [degrees]C (0.36 [.sup.0.F]) could be reached with the ON-OFF frequency of 6 times. The ON-OFF times of solenoid valve reduced obviously even under higher loading at 600 W (0.80 hp) without sacrificing the accuracy of temperature control. However, as the variation of temperature shown in Figure 4 (b), the cooling water temperature could reach the temperature of 20.4 [degrees]C(68.7 [.sup.0.F]) because of higher machining load and extension of valve opening duration at 15 seconds. It revealed that longer delay time would not be suitable for higher requirement of temperature control accuracy under larger machining load condition.

[FIGURE 4 OMITTED]

Effects of using capillary tube and thermostatic expansion valve (TEV) associated with the same PWM controlled time delay scheme have been examined extensively. Figure 5 (a) depicted the variation of cooling water temperature using TEV as a throttling device with delay time of 5 seconds under the loading of 200 W (0.27 hp). Experimental results revealed the control accuracy at 0.2 [degrees]C (0.36 [.sup.0.F]) could be achieved as well by applying the TEV as a throttling device. However, the ON-OFF frequency of 14 time because higher than the case of using capillary tube. Because TEV could adjust the mass flow rate of refrigerant automatically according to the superheat of evaporator, the ON-OFF times of by-pass valve would increase to compensate the heating load during lower loading condition. It would not be beneficial for the life span of solenoid valve while conducting the capacity control of hot-gas by-pass approach. Figure 5 (b) presented the variation of water temperature at time delay of 15 seconds under lower loading condition. The control accuracy of temperature at 0.2 [degrees]C (0.36 [.sup.0.F]) would not be obtained and the ON-OFF times were still higher than the case of using capillary tube. Furthermore, the similar trend could be presented through the experiments using a TEV as throttling device under the same delay time at 5 seconds and 15 seconds during higher matching loading condition at 600 W (0.80 hp). The variations of water temperature were display in Figure 6 (a) and Figure 6 (b), the ON-OFF frequency were 10 times and 7 times respectively. It revealed similar trend with Figure 5, but less ON-OFF times while using the TEV as a throttling device.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

Figure 7 depicts the variation of power consumption of machine tool cooler system using different throttling devices while conducting the hot-gas by-pass control scheme. Although the TEV did not present satisfactory results because of high frequency of ON-OFF operation (14 times for TEV and 10 times for capillary tube respectively), it demonstrated lower power consumption for the hot-gas by-pass system. The power consumption could be reduced about 25% for applying TEV than that of capillary tube. It might be due to the superheat of refrigerant was smaller for TEV system than capillary tube system. The TEV could maintain the superheat by adjusting the mass flow rate of refrigerant and resulted in small power consumption. While conducting the hot-gas by-pass scheme, the power consumption would decrease because of lower compression ratio caused by higher evaporating pressure and lower condensing pressure. It also revealed compromise between energy saving and durability of solenoid valve should be a trade-off while conducting different throttling devices for hot-gas by-pass scheme.

[FIGURE 7 OMITTED]

CONCLUSIONS

The main objective of this study is to evaluate the influence of the temperature control accuracy and durability of solenoid valve delay time control through PWM techniques for hot-gas by-pass scheme of a machine tool cooler system. The experimental investigation has been performed extensively by delay time control using PWM techniques under variation of machining load and different throttling devices. Temperature control accuracy as well as the durability of hot-gas by-pass solenoid valves was investigated through the measurement of the experimental rig. The experimental results revealed that the accuracy of temperature control at 0.2 [degrees]C (0.36 [.sup.0.F]) could be achieved through the extension of opening duration for 5 seconds with the ON-OFF frequency of 10 times under low machining load at 200 W (0.27 hp). However, longer delay time would not be suitable for higher requirement of temperature control accuracy under larger machining load condition. It also presented the strategy of delay time by PWM controller was feasible in reducing the ON-OFF frequency without sacrificing the control accuracy of 0.2 [degrees]C (0.36 [.sup.0.F]) especially under lower loading conditions. It also reveals that PWM controlled delay time control scheme along with adequate choice of throttling device can provide alternative option for steady temperature control and substantial energy-saving. The results from this study could provide valuable information to identify the feasibility on cost-effective way for capacity control in machine tool coolers without sacrificing the accuracy of temperature control.

ACKNOWLEDGMENTS

The authors would like to express their great appreciation to the financial support by the National Science Council under the grant No. NSC 98-2622-E-167-029-CC3. The financial support from the Bureau of Energy affiliated to Ministry of Economic Affairs and Industrial Technology Research Institute are also very much appreciated.

REFERENCES

ASHRAE Refrigeration Handbook. 2006. American Society of Heating Air Conditioning and Refrigerating Engineers. Georgia, USA.

Byund, J. S., J. H. Lee, and C. D. Jeonc. 2008. Frost retardation of an air-source heat pump by the hot gas bypass method. International Journal of Refrigeration, 31: 328-334.

Cho, H. H., Y. C. Kim, and I. K. Jang. 2005. Performance of a showcase refrigeration system with multi-evaporator during on-off cycling and hot-gas bypass defrost. Energy, 30: 1915-1930.

Choi, J. M., Y. C. Kim, and J. T. Chung. 2004. An empirical correlation and rating charts for the performance of adiabatic capillary tubes with alternative refrigerants. Applied Thermal Engineering, 24: 29-41.

Ding, G., and L. Fu. 2005 Performance analysis and improvement of air-to-water chiller for application in wide ambient temperature range. Applied Thermal Engineering, 25: 135-145.

Gao, Z. 2010. The impact of thermostatic expansion valve heating on the performance of air-source heat pumps in heating mode. Energy conversion and management, 51: 732-739.

Huang, D., Q. Li, and X. Yuan. 2009. Comparison between hot-gas bypass defrosting and reverse-cycle defrosting methods on an air-to-water heat pump. Applied Energy, 86: 1697-1703.

Ibrahim, G. A. 2001. Effect of sudden changes in evaporator external parameters on a refrigeration system with an evaporator controlled by a thermostatic expansion valve. International Journal of Refrigeration, 24: 566-576.

Lu, Y. Y., C. S. Shieh, and R. T. Hung. 2010. An improvement on the digital redesign method using fuzzy PWM controller. Journal of Process Control, 20: 709-715.

Mithraratne, P., N. E. Wijeysundera, and T. Y. Bong. 2000. Dynamic simulation of a thermostatically controlled counter-flow evaporator. International Journal Refrigeration, 23: 174-189.

Shih, M. C., and M. A. Ma. 1998. Position control of a pneumatic cylinder using fuzzy PWM control method. Mechatronics, 8: 241-253.

Taghizadeh, M., A. Ghaffari, and F. Najafi. 2009. Modeling and identification of a solenoid valve for PWM control applications. Comptes Rendus Mecanique, 337: 131-140.

Tso, C. P., Y. W. Wong, P. G. Jolly, and S. M. Ng. 2001. A comparison of hot-gas by-pass and suction modulation method for partial load control in refrigerated shipping containers. International Journal of Refrigeration, 24: 544-553.

Yu, F. W., K. T. Chan, and H. Y. Chu. 2006. Constraints of using thermostatic expansion valves to operate air-cooled chillers at lower condensing temperatures. Applied Thermal Engineering, 26: 2470-2478.

Yaqub, M., M. Syed Zubair, and K. Jameel-ur-Rehman. 2000. Performance evaluation of hot-gas by-pass capacity control schemes for refrigeration and air-conditioning systems. Energy, 25: 543-516.

Fu-Jen Wang, PhD, PE

Member ASHRAE

Jian-Wei Kao

Student Member ASHRAE

Fu-jen Wang is associate professor, Kuei-I Tsai is senior lecturer and Jian-Wei Kao is graduate student in the Department of Refrigeration, Air Conditioning and Energy Engineering, National Chin-Yi University of Technology, Taiwan. Hao-chung Lee is senior researcher in the Energy and environment Research Laboratories, Industrial Technology Research Institute, Taiwan.
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Article Details
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Author:Wang, Fu-Jen; Tasi, Kuei-I; Kao, Jian-Wei; Lee, Hao-Chung
Publication:ASHRAE Transactions
Article Type:Report
Geographic Code:9TAIW
Date:Jan 1, 2011
Words:3653
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