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an Investigation of Condensation Heat Transfer of Dowtherm a in Polymer Solidification

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Polymer which is used tire cord manufacturing, is transferred with jacketed transfer line and heating box to provide thermal stability of the system after extrusion process. The heating box is heated with Dowtherm-A vapor to prevent heat loss from the polymer. In order to increase condensation heat transfer, reducing the thickness of condensation is needed. In this study, inlet mass flow rate of Dowtherm-A was investigated with steady state analyses. Results show that fluid moves polymer pipes for higher inlet mass flow rate while it loses its energy and faces towards solidification chamber for lower inlet mass flow. In order to understand the effect of surface inclination over fluid film thickness two different solidification chamber designs were analysed. Heat transfer rate is improved with inclined top surface in comparison to flat top surface. Lower fluid film thickness increases heat transfer rate. Therefore, heat transfer rate of solidification chamber of inclined top type is %3.5 higher than flat top type. It is also investigated that approximately %46,1 of the heat is transferred to solidification chamber and %3,9 of the heat transferred to polymer pipes whereas another half of the heat is lost due to heat transfer with ambient air.

Introduction

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A heat transfer fluid is a mixture of two organic compounds, biphenyl (C12H10) and diphenyl oxide (C12H10O). These compounds have practically the same vapor pressures, so the mixture can be handled as if it were a single compound. Its normal application range is 15°C to 400°C, and its pressure range is from atmospheric to 10.6 bar. The fluid may be used in systems employing either liquid phase or vapor phase heating. Four important properties that help determine the viability of a heat transfer fluid in a particular application are stability, vapor pressure, freeze point, and viscosity. Vapor phase heating has an advantage where it is difficult to liquid flow pattern and velocity and no pumps are needed when a gravity return condensate system is used with a natural circulation vaporizer.

It is very importat issue to keep preffered temperature value of polymer. It affects directly orientation of the polymer and consequently affects yarn’ properties. Using with this heat transfer fluid, heat is transferred at the saturation temperature of the vapor. As a result, such units can provide uniform, recisely controlled temperatures. Dowtherm-A vapor phase systems provide much more heat per unit mass of heat medium passed through the user for low pressure values. According to these adventages, Dowtherm-A heat transfer fluid is used for in this study.

Condensation may occur either in dropwise or filmwise condensation. Regardless the type of condesnation whether it is dropwise or film, heat resistance of condensation is increases with film thickness. Therefore, reducing the thickness of condensation increases heat transfer rate. Another way to increase heat transfer is using surface coatings that inhibit wetting .

Two-phase cycle provides to transfer high rate of heat between the surfaces with almost zero temperature difference along the surfaces. Two-phase Dowtherm-A was investigated and heat transfer performance was increased with inclination angle to 5-degree for a horizontal two-phase loop thermosiphon (HLTS). A study for heat pipes completed by Anderson shows that Dowtherm-A provides adequate thermal stability and compatibility with 304 SS for 350⁰C. Another study investigates that Dowtherm-A operates within temperature range 200-450⁰C in a wickless heat pipe with shell material of 316 SS.

Heating Box System

Polymer is transferred with jacketed transfer line and heating box to provide thermal stability of the system after extrusion process. In normal operation, heat generated by the meter pump is adequate to maintain operating temperature. When the meter pump is shut down, polymer would freeze in the unit without the heating jacket. For this reason we need a heat transfer medium to keep preffered temperature value of polymer.

The heating box is heated with Dowtherm-A vapor which is enters from Dowtherm-A inlet to prevent heat loss from the polymer as shown in Figure 1. It is covered and temperature is maintained by this heat transfer fluid. Dowtherm-A in heating box system entirely condenses and leaves from the Dowtherm-A outlet as liquid phase. Also, heating box has a vent line for unhandled exceptionsi Temperature is controlled by adjusting Dow pressure and a thermocouple is used to measure Dowtherm fluid temperature. High and low pressure alarms are provided. And fluid temperature and pressure values are indicated in the control rooms.

CFD & CHT Analysis

CFD & CHT analysis were performed for heating box system. Star CCM+ is utilized to model the CFD&CHT simulation of heating box system. The heating box system is composed of gas phase Dowtherm-A inside the box, liquid phase of polymer inside the pipes, liquid phase Dowtherm-A on condensation surfaces and solid phase solidification chamber, pipes and insulation material in CFD model.

Dowtherm-A enters to system as gas phase with 300⁰C. Temperature decreases and condensation occurs in outer surfaces including pipes before and after pump, solidification chamber; inner surface of insulation. Several assumptions were made during CFD&CHT analysis: 1) Dowtherm-A in heating box system entirely condenses and leaves the box as liquid phase for transient analysis. 2) Since velocity of polymer is very low in solidification chamber, it is assumed that advection term is negligible. Therefore it is modelled as solid 3) Temperature change in pump and pipes outside of heating box is neglected.

Firstly, steady state CFD analyses were performed to investigate the effect of inlet velocity to system. Condensation was not involved steady state analyses. Secondly, the inlet mass flow is calculated for the heating box system which heat enters to system is equal to heat loss of system. With this calculation, polymer temperature is stabilized in transient CFD analyses and test system. Finally, transient CFD analyses were completed to include condensation in CFD models.

 Steady State Analysis

Fluid tends to move polymer pipe with high energy in higher inlet mass flow rate cases. However, it loses its energy and faces towards solidification chamber for lower inlet mass flow.

Mass flow inlet is calculated with respect heat loss of dow heating box system. Total heat loss of heating box is equal to temperature difference divided by equivalent resistance. Figure 6 shows the heat transfer loss walls and their resistances due to convection and conduction.

Resistances were calculated with convection and conduction heat transfer in Eq (1):

R_1= 0 R_2= L_casing/(k_casing A_casing ) R_3= L_insulation/(k_insulation A_insulation ) R_4= 1/(h_air A_outer ) (1)

R_eq= R_1+R_2+R_3+R_4 (2)

Total heat loss was calculated as Eq(3):

Q ̇= ∆T/R_eq (3)

 Transient Analysis with Condensation

 Required mass flow for heat equilibrium is calculated 0.0035 kg/s which is defined as mass flow inlet for transient analysis. The model consists of gas and liquid phase Dowtherm-A, liquid phase polymer and solid phase heating box casing, insulation and solidification chamber domains. Liquid phase polymer enters the system with pre-pump pipe and a pressure outlet is defined before pump. Liquid phase polymer enters the system with post-pump pipe again and its pressure is higher. A mass flow outlet is defined before solidification chamber for post-pump pipe.

Conclusion

In this paper the effect of inclination top surface of solidification chamber was investigated with two different dow heating box designs. Results show that inclined top surface remove more fluid from top surface and decreases fluid film thickness. Lower fluid film thickness increases heat transfer rate. Therefore, heat transfer rate of solidification chamber of Design 2 is %3.5 higher than Design 1. It is also investigated that approximately %50 of the heat is transferred to solidification chamber and polymer pipes whereas another half of the heat is lost due to heat transfer with ambient air. Total heat transfer is 1008 Watt for the dow heat box.

In steady state cases, fluid moves toward polymer pipe with high energy in higher inlet mass flow rate cases. However, it loses its energy and faces towards solidification chamber.Required Dowtherm-A mass flow for heat equilibrium is calculated 0.0035 kg/s for this system. It is defined as mass flow inlet for transient analysis. Therefore, heating box reaches a steady state equilibrium at approximately 300⁰C for transient analysis.

References

  1.  “Dowtherm-A Heat Transfer Fluid-A Product Technical Data”, Dow Chemical Company, USA, 1997.
  2.  F.P. Incropera, D.P. DeWitt, T.L. Bergman, A.S. Lavine, Fundamentals of Heat and Mass Transfer, 6th Edition.
  3. A. Faghri, “Heat Pipe Science and Technology”, Taylor&Francis, 1995.
  4. Y.Wang, X. Wang, H. Chen, H. Fan, R.A. Taylor, Y. Zhu, “CFD Simulation of an Intermediate Temperature, Two-phase Loop Thermosiphon for Use as a Linear Focus Solar Receiver” Energy Procedia, 2017, vol. 105, pp. 230 – 236.
  5.  W.G. Anderson et al., “Intermediate temperature fluids life tests–experiments” Proceedings of the 16th International Heat Pipe Conference, Lyon, France, 2012.
  6.  H. Jouhara, A.J. Robinson, “An experimental study of small-diameter wickless heat pipes operating in the temperature range 200⁰C to 450⁰C” Heat Transfer Engineering, 2009, 30: 1041-1048. 
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