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### 97 rajesh

1. 1. IV th International Conference on Advances in Energy Research Indian Institute of Technology Bombay, Mumbai P ro c e s s C o n t ro l S t r a t e g y A n d I t s I m p a c t O n Performance Of The Cold Box Of G u a r d e d H o t B o x Te s t F a c i l i t y F o r U - v a l u e M e a s u re m e n t Debasish Chowdhury, Rajesh Chatterjee, Subhasis Neogi School of Energy Studies Jadavpur University
2. 2. School of Energy Studies Jadavpur University OUTLINE  Introduction  Guarded Hot Box Test Facility  Experimental set-up  Result and discussion  Conclusion
3. 3. School of Energy Studies Jadavpur University INTRODUCTION Recent times have seen a tremendous increase in building energy consumption. Therefore, reducing this load becomes one of the most effective ways to conserve energy in buildings. U-value of any material can be used as a tool for thermal characterisation of any material. Use of low U-value building material thus save energy A Guarded Hot Box Test Facility is used to measure the overall heat transfer coefficient of any material. 3
4. 4. School of Energy Studies Jadavpur University U-value can be defined as: Lj 1 h ce 1 h re n j 1 k j 1 h ci h ri where, h is heat transfer coefficient. c & r refer to convection and radiation respectively. i & e refer to internal and external environment. k is thermal conductivity of layer or layers having thickness L. 4
5. 5. School of Energy Studies Jadavpur University GUARDED HOT BOX TEST FACILITY Fan Metering Box Plate Heater Fan Baffle Specimen Heater Fan HE-Fan Coil Unit Guard Box Cold Box Baffle Heater Surround Panel Schematic view of Guarded Hot Box Test Facility complying with BS EN ISO 8990:1996 5
6. 6. School of Energy Studies Jadavpur University The Guarded Hot Box design is based on BS EN ISO 8990:1996. It essentially consists of metering box which is enveloped by a guard Box and cold Box. Sandwiched between the two chambers is a surround panel holding the sample. Walls of Guarded Hot Box including the Surround Panel are made of extruded polystyrene blocks. Purpose of guard box is to limit heat transfer via Metering Box wall. Baffle plate is placed parallel to the surface of test element in both the boxes. 6
7. 7. School of Energy Studies Jadavpur University Virtual PID based controller maintains constant temperature in guard box and metering box. Traditional air condition system using thermostatic control is avoided. Chilled ethyl-glycol based storage system incorporating a Heat Exchanger-Fan coil unit is used. A simple PID controller is used to turn on and off the circulation of chilled fluid (via relay) through the heat exchanger to maintain constant temperature inside the cold box. The purpose of the heat exchanger provided is to continuously remove the heat entering into the cold box for maintaining appropriate temperature conditions. 7
8. 8. School of Energy Studies Jadavpur University Temperature measurement involves use of K-type thermocouples. Thermocouples measure temperature of air, baffle and surface of sample in both Metering Box and Cold Box. AGILENT 34970A Data logger Air temperature values from thermocouples located inside the guard box and metering box are averaged respectively to form input signal to the virtual PID controller 8
9. 9. School of Energy Studies Jadavpur University Out of three sensors located in the air curtain in front of the sample inside the cold box, the sensor located is used as input sensor to controller. Steady state thermal transmittance is calculated by measuring the heat flux the specimen and measuring the air temperature in Cold and Metering Box. U Q A T W /(m 2 K )
10. 10. School of Energy Studies Jadavpur University Metering Box Surround Panel Baffle Guard Box Power Supply Unit Cold Box Guarded Hot Box Test Facility at School of Energy Studies 10
11. 11. School of Energy Studies Jadavpur University Surround Panel containing the Sample Cold Box Guard Box surrounding the Metering Box Components of Guarded Hot Box Test Facility 11
12. 12. School of Energy Studies Jadavpur University EXPERIMENTAL SET-UP The experimental set-up consists of : Heat exchanger – fan or Fan-Coil assembly, COLD BATH-35 make chilling plant, Circulating pump along with interfacing circuit for controlling circulation of chilled ethyl glycol inside the heat exchanger. Schematic overview of control circuit 12
13. 13. School of Energy Studies Jadavpur University COLD BATH-35 make chilling plant Heat exchanger – fan or Fan-Coil assembly Relay PID Controller Line terminal of Circulating pump Interfacing circuit for controlling circulation of chilled ethyl glycol inside the heat exchanger. 13
14. 14. School of Energy Studies Jadavpur University Control strategy implemented by the controller is actually Pulse Width Modulation or PWM control. By varying the duty cycle (% of Cycle time for which relay is ON), the rate of heat removed by heat transfer fluid is controlled. The controller compares the set value (SV) with process value (PV) i.e. the cold box air temperature and turns on or off the circulation as required. 14
15. 15. School of Energy Studies Jadavpur University Time temperature profile of air temperature inside the guard box, metering box and cold box. 15
16. 16. School of Energy Studies Jadavpur University Diagram showing varying duty cycle of relay Zone 1: Duty cycle is 100% Zone 2: Duty cycle is 80% Zone 3: Duty cycle is 60% Zone 4: Duty cycle is 40% Zone 5: Duty cycle is 20% 16
17. 17. School of Energy Studies Jadavpur University The set point for metering box and guard box was set at 40°C and that of cold box was set at 0°C. The effect of varying the parameters of the controller namely Proportional Band (P), Integral Time constant (Ti)and Derivative Time constant (Td) on cold box air temperature profile is studied. The surround panel is provided with a sample fixing window of 500 x 500 mm where an insulation sheet of 50 mm thickness is used as sample case. 17
18. 18. School of Energy Studies Jadavpur University RESULTS & DISCUSSION Cold box air temperature profile for default values of controller 18
19. 19. School of Energy Studies Jadavpur University • Temperature profiles in case 1 and case 2 are almost similar. • Derivative action is required to damp out rapid changes in process value • In other words it has a stabilization effect on the system. • Structural heat load, cooling load due to circulating fans and leakages create an inherent stabilization effect. 19
20. 20. School of Energy Studies Jadavpur University 20
21. 21. School of Energy Studies Jadavpur University • Temperature profiles in case 3 and case 4 are also similar. • But steady state error in case 3 is more than case 2. • This is because control action steps in much closer to set point in case 4 where proportional band is kept at 1ºC. 21
22. 22. School of Energy Studies Jadavpur University In proportional control, control action takes place only when process value (PV) enters the Proportional Band (PB). Below & above the PB, no control action takes place. Proportional action is also sluggish in nature and leaves a resultant steady state error. This error can be reduced by decreasing PB but can be eliminated by integral action only. This is evident from the case studies above. In case 1 and case 2, controller operates in PID and PI mode respectively. It is expected that both of them result in similar nature of response. 22
23. 23. School of Energy Studies Jadavpur University The only dissimilar is being that : For case 1: (Controller operated in PID mode) Steady state average value is 0.15°C. Minimum temperature reached was -0.890°C after about 1.088 hours from start. For case 2: (Controller operated in PI mode) Steady state average value is 0.262°C. Minimum temperature reached was -0.262°C after about 1.685 hours from start. 23
24. 24. School of Energy Studies Jadavpur University This can be explained from heat exchanger fluid temperature profiles of two cases. 35 Ethyl Glycol Inlet Temperature Ethyl Glycol Outlet Temperature Air Outlet Temperature Air Inlet Temperature 25 15 Ethyl Glycol Inlet Temperature Ethyl Glycol Outlet Temperature Air Outlet Temperature Air Inlet Temperature 25 TEMPERATURE (oC)  TEMPERATURE (oC)  35 15 5 -5 -15 5 -5 -15 -25 0 0.5 1 TIME IN HOURS  1.5 2 Time temperature profile of heat exchanger fluids for Case 1 -25 0 0.5 1 TIME IN HOURS  1.5 Time temperature profile of heat exchanger fluids for Case 2 24 2
25. 25. School of Energy Studies Jadavpur University CONCLUSION In this paper a PID based controller was designed for maintaining a constant air temperature inside the cold box. Experimental results showed that when the controller was operated in PI and PID mode, the steady state air temperature achieved inside the cold box was closest to the set point. Moreover while operating in P mode, steady state error is more in the case where proportional band is larger Thus from the above experimental results it is evident that in the present cooling strategy implemented, varying the parameters of the controller has a significant effect on the cooling performance. 25