Knudsen, Hans Jorgen Hogaard

Langmaack, Lasse Nicolai

English

The bulb of a thermostatic expansion valve (TXV) is basically a temperature-pressure converter. It senses the temperature at the outlet of the evaporator, and the substance in the bulb (charge) generates the corresponding saturation pressure inside the bulb. The bulb is mounted on the evaporator outlet with a special mounting strap. The heat transfer is quite complex because it takes place both directly through the contact points between bulb and pipe and indirectly through the mounting strap The TXV has to react to temperature changes at the evaporator outlet. Therefore, the dynamic behavior of the valve (and thereby the whole refrigeration system) depends greatly on the heat transfer between the evaporator outlet tube and the charge in the bulb. In this paper a model for the overall heat transfer between the pipe and the charge is presented. Geometrical data and material properties have been kept as parameters in order to be able to see the effect of changes in those. Some of the parameters (e.g. thermal contact resistances) have been determined by a finite element model and a series of experiments. The model has been validated using test results obtained under different operating conditions and has been found to predict the time constant for the temperature development in the bulb within 1-10 %. Furthermore it has been found that app. 20% of the heat transfer takes place trough the mounting strap

Knudsen, Hans-Jørgen Høgaard

Online Research Database In Technology

Heat transfer from the evaporator outlet to the charge of thermostatic expansion valves

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Purdue UniversityPurdue e-PubsInternational Refrigeration and Air ConditioningConference School of Mechanical Engineering2006Heat Transfer From the Evaporator Outlet to theCharge of Thermostatic Expansion ValvesLasse Nicolai LangmaackTechnical University of Denmark and Danfoss A/SHans Jorgen Hogaard KnudsenTechnical University of DenmarkFollow this and additional works at: http://docs.lib.purdue.edu/iraccThis document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu foradditional information.Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/Herrick/Events/orderlit.htmlLangmaack, Lasse Nicolai and Knudsen, Hans Jorgen Hogaard, "Heat Transfer From the Evaporator Outlet to the Charge ofThermostatic Expansion Valves" (2006). International Refrigeration and Air Conditioning Conference. Paper 821.http://docs.lib.purdue.edu/iracc/821 R009, Page 1   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006   			  Lasse Nicolai Langmaack1*, Hans Jørgen Høgaard Knudsen2 1M. Sc., Industrial PhD- student Energy Engineering Section Dept. of Mechanical Engineering Technical University of Denmark   Danfoss A/S, R&D lab. Nordborgvej 81, Denmark Tlf.: +45 7488 4950  Fax.: +45 7488 5635 Mail: Lasse_L@Danfoss.com 2Associate Professor  Energy Engineering Section Dept. of Mechanical Engineering Technical University of Denmark Nils Koppels Allé, Building 402 DK-2800 Kgs. Lyngby, Denmark  Phone:  +45 4525 4121 Fax:      +45 4593 5215 E-mail:   hk@mek.dtu.dk   ABSTRACT  The bulb of a thermostatic expansion valve (TXV) is basically a temperature-pressure converter. It senses the temperature at the outlet of the evaporator, and the substance in the bulb (charge) generates the corresponding saturation pressure inside the bulb. The bulb is mounted on the evaporator outlet with a special mounting strap. The heat transfer is quite complex because it takes place both directly through the contact points between bulb and pipe and indirectly through the mounting strap  The TXV has to react to temperature changes at the evaporator outlet. Therefore, the dynamic behavior of the valve (and thereby the whole refrigeration system) depends greatly on the heat transfer between the evaporator outlet tube and the charge in the bulb.  In this paper a model for the overall heat transfer between the pipe and the charge is presented. Geometrical data and material properties have been kept as parameters in order to be able to see the effect of changes in those. Some of the parameters (e.g. thermal contact resistances) have been determined by a finite element model and a series of experiments. The model has been validated using test results obtained under different operating conditions and has been found to predict the time constant for the temperature development in the bulb within 1-10 %. Furthermore it has been found that app. 20% of the heat transfer takes place trough the mounting strap.  The work is part of the development of a complete model for different types of charges for TXV’s.  1 INTRODUCTION  Thermostatic expansion valves (TXV) have been used as regulation devices in refrigeration systems for many years. The main purpose of the TXV is to meter the flow of refrigerant into the evaporator in order to maintain a certain superheat at the evaporator outlet. Furthermore the TXV divides the high pressure side from the low pressure side of the refrigeration plant. The bulb of the TXV is mounted at the outlet of the evaporator where it senses the temperature. In the static situation, the temperature of the bulb will stabilize somewhere between the ambient temperature and the temperature of the evaporator outlet tube. In the dynamic situation however, it is important to know the dynamics of the temperature response. In the literature known to the author, there have only been a few investigations on the dynamic temperature response of the charge for a TXV. The work closest related was published by James & James (1987). They developed a mathematical model for a TXV. Part of that model describes the dynamics of the charge. Unfortunately they did not quantify the thermal resistance between the bulb wall and the evaporator tube. Also they did not take the strap with which the bulb is mounted into account. During the work presented in this paper a mathematical model for the temperature response of the charge was developed. The model is a lumped mass model which includes the heat transfer of the mounting strap. The mathematical model has been validated through experimental tests under various conditions.  R009, Page 2   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  2 THE MATHEMATICAL MODEL  The bulb is mounted on the evaporator tube as shown in Figure 1.  Changing the surface temperature of the evaporator outlet, will cause the temperature of the charge to change. Over time the temperature of the charge will stabilize at a temperature between that of the evaporator outlet and the ambient temperature. In the present work, this response is treated as a first order system. The response of a first order system to a unit step change is described by Equation 1.   τ−−=te1)t(y  Equation 1 (Raven (1996)  The model needed is a model that describes the temperature development in the charge as a reaction of a change of the surface temperature of the evaporator outlet tube. In order to describe the heat flow in the construction some definitions have to be presented. The construction can be divided into 8 parts as shown in Figure 2.     Figure 2: Definition of thermal system Figure 3: Heat fluxes in the system  The heat flows in the system are shown in Figure 3. In a more schematic way, the heat fluxes can be illustrated as shown in Figure 4.   Figure 4: Heat fluxes in the system   Figure 1: Bulb mounted on evaporator tube  Strap Evaporator tube Bulb 1 2 3 Amb 4 5 6 7 Where:  y :Response t  :time ι :Time constant Q2-amb  R009, Page 3   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  From Figure 4 it can be seen that the temperature of the charge is a result of two different series of heat fluxes in the system. They are:   1. Refrigerant (1)  Evaporator tube wall (2)  bulb wall (6)  Charge (7)  2. Refrigerant (1)  Evaporator tube wall (2) Strap et (3) Free strap (4) Strap bulb (5)  bulb wall (6)  Charge (7)  In order to simplify the modeling, the following assumptions are made:  • Lumped mass model • No temperature profile through or along the perimeters of walls. • Strap has no mass • The strap is modeled as a fin where the temperatures at the endpoints are known.  Heat flux in the above system:  21i_etref21 TAhQ −− ∆⋅⋅=  Equation 2  62c62 TR1Q62−− ∆⋅=−  Equation 3  62c32 TR1Q32−− ∆⋅=−  Equation 4  amb2strap_etairamb2 TAhQ −− ∆⋅⋅=  Equation 5  )Lmsinh()Lmcosh(M2Qfree_strapblfree_strap43 ⋅θθ−⋅⋅⋅=−  Equation 6  )Lmsinh(1)Lmcosh(M2Qfree_strapfree_strapbl54 ⋅−⋅θθ⋅⋅=−  Equation 7  65c65 TR1Q65−− ∆⋅=−  Equation 8  amb5strap_bairamb5 TAhQ −− ∆⋅⋅=  Equation 9  76i_beargch76 TAhQ −− ∆⋅⋅=  Equation 10  amb6amb_free_bairamb6 TAhQ −− ∆⋅⋅=  Equation 11  43amb332 QQQ −−− +=   Equation 12   R009, Page 4   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  65amb554 QQQ −−− +=   Equation 13  5443amb4 QQQ −−− −=   Equation 14  amb5l TT −=θ  Equation 15  amb3b TT −=θ  Equation 16  cross_strapstrapstrapair2APhm⋅λ⋅=  Equation 17  ( )amb2cross_strapstrapstrapair TTAPhM −⋅⋅λ⋅⋅=  Equation 18  Combining these heat fluxes to describe the three ways the bulb is affected, results in following equation system:  62322122p2 QQQdtdTcm −−− −−=⋅   Equation 19  amb676656266p6 QQQQdtdTcm −−−− −−+=⋅   Equation 20  7677p7 QdtdTcm −=⋅   Equation 21  3 THERMAL RESISTANCES  The model presented above includes three thermal resistances Rc 2-6, Rc 2-3 and Rc 5-6. These have been determined by the help of experimental tests and a finite element model. In the following these tests and the finite element model will be presented. The resulting thermal resistances will be inserted into the analytical model which then again will be compared to test results.  3.1 Experimental evaluation of the temperature distribution in empty bulb  In the simulations, the contact resistances will be the unknown values to determine. Therefore the temperature distribution in the bulb wall needs to be known. This has been done experimentally by measuring the temperature at four points inside an empty bulb. The temperature was measured at four points as shown in Figure 5. The measured temperature curves have been used to calibrate the FE model.  3.2 Test series The test series covers tests of the two contacts individually and a combination. Three tests were performed with conditions as shown in Table 1.    Figure 5: Temperature sensors  Pipe contact (Contact 1) Strap contact (Contact 2) Test 1 X  Test 2  X Test 3 X X Table 1: Tests performed on bulb with temperature sensors Bulb Evap. tube T3 T4 T1 T2  R009, Page 5   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006   The tests show the temperature distribution in the bulb.  3.3 Results  Test 1 Test 2 Test 3  Figure 6: Wooden brick as insulation between strap and bulb  Figure 7: Wooden brick as insulation between pipe and bulb  Figure 8: Full contact Sensor Time constant Sensor Time constant Sensor Time constant 1 144 1 48 1 70 2 120 2 57 2 43 3 10 3 149 3 9 4 122 4 57 4 63  3.4 The Finite element model The finite element (FEM) software used for this investigation is SORPAS®. For simplicity the problem is modeled as a 2- dimensional problem. Figure 9 and Figure 10 show the real system and the modeled section respectively.  The FE model looks slightly different from the real model. The differences are partly caused by modeling limitations and partly due to numerical issues.  3.5 Boundary conditions For a SORPAS® model it is required to model tools where the mechanical boundary conditions can be applied. The mechanical boundary condition for this model is an initial movement of the bulb towards the pipe. In the Danfoss refrigeration laboratory it has become common practice to tighten the strap until the bulb has moved 1 mm into the pipe, which results in a deformation of the copper tube. This is also done in the simulations. The thermal boundary condition is that the copper tube keeps its temperature constant during the simulation. It has been chosen to set the temperature of the copper tube to 0ºC while all other parts have an initial temperature of 5ºC. Ambient temperature is set to 5ºC but the model is considered as being insulated from the ambient.  3.6 Calculations The contact between two parts will never be as perfect in reality as it can be modeled theoretically. In order to compensate for this difference, interface layers are used.  The interface layer is a material with a given conductivity and with very low heat capacity which is put in between the real contact faces. The thickness and conductivity of the interface layer will determine the heat flow between the two parts.   Figure 9: Real system Figure 10: Modelled section Section  R009, Page 6   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  Basically this model needs two different interface layers, one between the bulb and the pipe (contact 1) and one between the strap and the bulb (contact 2). The thickness of both has been set to 0.5 mm. Following procedure was used to find the conductivity of them  Initially a number of calculations for different conductivities of the interface layers were performed. The conductivity of one interface is varied while the other is set to 0.001 W/m/k. These analyses are comparable with the experimental results where one contact is insulated while the other is not.  3.7 Results For the contact 1, it was found that a conductivity of 0.75 W/m/K for the interface layer gave the results closest to the experimental results. Whereas for the contact 2 it was found that a conductivity of 0.075 W/m/K gave the closest results. Table 2 shows the results obtained.   Measured  FEM Deviation Deviation in % Contact 1     Sensor 1 144 167 23 16 Sensor 2 120 117 -3 -2.5 Sensor 3 10 10 0 0 Contact 2     Sensor 1 48 45 -3 -6 Sensor 2 57 65 8 14 Sensor 3 149 110 -39 -26 Table 2: Comparison of time constants [s] for increasing temperature  3.8 Thermal resistance The thermal resistance R is defined as:  tAlR ⋅⋅λ=  Equation 22 (Incorpera and DeWitt (1996))  Thus the thermal resistances R can be found:  Contact Area [m2] λ [W/m/K] t [m] R [K/W] Bulb/pipe 7.40E-5 0.75 0.0005 9 Strap/bulb 4.787E-4 0.075 0.0005 13.9 Strap/evaporator tube 5.721E-4 0.075 0.0005 11.6 Table 3: Thermal resistances   3.9 Conclusion on the finite element simulations The finite element calculations have given an estimate of the thermal resistances of the two contacts. These can now be used in the analytical lumped mass model. For further verification the results from the analytical model have been compared to experiments with charged bulbs where the pressure of the bulb was measured.              R009, Page 7   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  4 THE ANALYTICAL MODEL  The contact resistances can be inserted in a previous modeled lumped mass model of the charged system presented in section 2.  Inserting the given conditions, the model should be able to predict the time constants as measured on the charged bulbs. In order to see the effect of each of the two contacts, three comparisons are made. One for only pipe contact (insulated strap), one for only strap contact (insulated pipe), and one in which both contacts are active.  Furthermore one comparison is made for a larger amount of charge also with both contacts active.  4.1 Test on charged bulbs Some tests were performed on bulbs charged with a very small amount of charge (80 mg Propane (R290)). The bulbs were mounted in four different ways. Some were mounted on the top of the pipe, where the liquid charge will be close to the pipe and some underneath the pipe, where the liquid charge will be close to the strap.    Mounted on top Mounted underneath Strap insulated Pipe insulated Bulb 1 X   X Bulb 2 X  X  Bulb 3  X X  Bulb 4  X  X Table 4: Mounting of the four bulbs  Results:  Time constant up Time constant down Bulb 1 245 122 Bulb 2 75 15 Bulb 3 219 103 Bulb 4 127 33 Table 5: Time constants for the four bulbs  The right comparison for the time constant going upwards can be made between bulb 2 for Contact 1 and bulb 3 for contact 2. For the time constant going downwards on the other hand, the right comparison can be made between bulb 1 for insulated pipe and bulb 4 for insulated strap.   Strap insulated Pipe insulated Scale Time constant up 75 219 1:2.92 Time constant down 33 122 1:3.7 Table 6: Comparable time constants for the two contact interfaces  4.2 Results  Test Model Deviation in % R290 80 mg, strap contact (insulated pipe) 219 232 6 R290 80 mg, only pipe contact (insulated strap) 75 77.9 4 R290 80 mg (both contacts) 52 58.5 12.5 R290 1630 mg (both contacts) 62 62.5 0.8 Table 7: Comparison of time constants from tests and calculations         R009, Page 8   International Refrigeration and Air Conditioning Conference at Purdue, July 17-20, 2006  5 CONCLUSION AND FURTHER WORK  Using the thermal resistances found by Test/ FEM, the model predicts the time constants for the two systems with an accuracy of 1-13 %. Furthermore, the measured time constants show that the direct contact between bulb and evaporator pipe transfers 75-80% of the heat, while the strap transfers the remaining 20-25%.  This work is part of a project with the aim to develop a model for the static and dynamic behavior of charges for TXV. Therefore, this work needs to be extended to include charges with thermal ballast brick and/ or non- condensable gas.  NOMENCLATURE  href  : Heat transfer coefficient of refrigerant  [W/m2/K] hcharge  : Heat transfer coefficient of charge   [W/m2/K] Rc_2-6  : Contact resistance between etube/bulb [m2 K / W] Rc_2-3  : Contact resistance between etube/strap [m2 K / W] Rc_5-6  : Contact resistance between strap/bulb [m2 K / W] Pstrap  : Perimeter of strap   [m] hair  : Heat transfer coefficient of air  [W/m2/K] λstrap  : Heat conductivity of strap  [W/K/m] Aet_strap  : Contact area between strap and evap tube [m2] Ab_strap  : Contact area between strap and bulb [m2] Ab_free_amb : Part of bulb surface that is free to ambient [m2] Aet_i  : Inner area of evaporator tube  [m2] Ab_i  : Inner area of bulb   [m2] Astrap_cross : Cross sectional area of strap  [m2] Lstrap_free  : Length of free strap   [m]  REFERENCES  1. Incropera, F. P., De Witt, D. P., 1996, Introduction to heat transfer, John Wiley and sons, Inc., USA, 801 pages. 2. James, K.A., James R.W., 1987, Transient analysis of thermostatic expansion valves for refrigeration system evaporators using mathematical models, Trans. Inst. M C Vol. 9 No. 4. 3. Raven H. F., 1995, Automatic Control Engineering, McGraw Hill, Singapore, 619 pages.  

Heat Transfer From the Evaporator Outlet to the Charge of Thermostatic Expansion Valves

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Heat Transfer From the Evaporator Outlet to the Charge of Thermostatic Expansion Valves

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