1、 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 Re
2、search Laboratories, Industrial Technology Research Institute, Taiwan. Experimental Investigation of a Machine Tool Cooler Using Hot-Gas By-Pass Valves for Temperature Control Fu-Jen Wang, PhD, PE Kuei-I Tasi Jian-Wei Kao Hao-Chung Lee Member ASHRAE Student Member ASHRAE ABSTRACT HEADING Machine too
3、l with more accurate, stable and higher precision for the machining components cannot be developed without the precise control of the temperature rise by appropriate cooling. The machine tool coolers are the best managers of oil (or water) temperature in avoiding the deviation of spindle centerline
4、for machine tools. However, the machine tool coolers are facing the control hunting of cooling temperature and the dramatic variation of heat load during high speed machining. In this study, the experimental investigation using hot-gas by-pass scheme were conducted to evaluate the temperature contro
5、l accuracy of process cooling water. Effects of delay time control use pulse width modulation (PWM) techniques for by-pass valve under variation of process cooling load and different throttling devices been have investigated. The accuracy of temperature control as well as the frequency of ON-OFF tim
6、es under different loading and different delay time has been compared comprehensively. The experimental results revealed that control accuracy within 0.2W (0.36 0F) of cooling water temperature can be achieved with 5 seconds delay time controlled by adjusting hot-gas by-pass valve. The proposed hot-
7、gas by-pass cooling system with PWM controlled time delay scheme is not only more cost-effective than inverter-driven system, but also can provide higher durability for by-pass solenoid valve and perform precise temperature control specific for high-accuracy process cooling application. INTRODUCTION
8、 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 avoidi
9、ng 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 tempe
10、rature. 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 va
11、lves 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). Yaq
12、ub 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 LV-11-C022178 ASHRAE Transactions2011. American Society of Heating, Refrige
13、rating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions, Volume 117, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAES prior written permission.study was also p
14、resented 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 metho
15、d 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 meth
16、ods 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 us
17、eful 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 temperatur
18、e 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 c
19、ost-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 step
20、per 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 contr
21、ol 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 considera
22、tion 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,
23、 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-
24、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 impr
25、oving 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
26、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 sys
27、tem 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 temperat
28、ure 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 exte
29、nsively. 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 contr
30、ol. EXPERIMENTAL DEVICES AND TESTING System Description 2011 ASHRAE 179The 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 devi
31、ces, 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 machi
32、ne 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
33、. 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 g
34、lasses 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 ta
35、nk, 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
36、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. Two high quality RTD (PT-100) sensors with an uncertainty of
37、 0.1 (0.18 0F) 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 0.05p of reading. The power consumption of
38、 compressor was measured on-site by a power meter (HIOKI, model 3169-20) with an uncertainty of 0.01p 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 perfo
39、rmed 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 to 45 ! (32 0F to 113 0F) with an uncertainty of 0.5 ! (0.9 0F) controlled by PID controller, and relative humidity ranging f
40、rom 30% - 90% RH with an uncertainty of 3%. Figure 1 Schematic layout of the experimental rig for machine tool cooler 180 ASHRAE TransactionsFigure 2 Schematic block diagram of PWM scheme for time delay control of hot-gas by-pass valve Hot-gas By-pass Control Loop The hot-gas by-pass loop consists o
41、f 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 d
42、ecreased, 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 co
43、mparative 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
44、 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)
45、signal to drive the hot-gas by-pass solenoid to modulate the temperature of the cooling water of the machine tool cooler. 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
46、 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 operati
47、on 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 experi
48、mental 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 variatio
49、n 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 by-pass solenoid valve were conducted under low machining
