University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat ExchangerUniversity of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 1/9 Motivation The experiment will introduce 1. Various types of heat exchangers, similarities and differences between them. 2. The mechanism of heat exchange, temperature variation in a heat exchanger. 3. Ideal heat exchanger and its comparison with the various types available forms 4. Effectiveness and the way it is calculated. 5. Effectiveness of different configurations of exchanger. 6. Dependence of effectiveness on various parameters. Physical and logical constraints why the ideal heat transfer cannot be achieved. Introduction A heat exchanger is a device by which thermal energy is transferred from one fluid to another. The types of heat exchangers to be tested in this experiment are called single-pass, parallel-flow and counter-flow concentric tube heat exchangers. In a parallel-flow heat exchanger, the working fluids flow in the same direction. In the counter flow exchanger, the fluids flow in parallel but opposite directions. (See Figure 1) Figure 1, Concentric Tube Heat Exchangers The variables that affect the performance of a heat exchanger are the fluids’ physical properties, the fluids’ mass flow rates, the inlet temperature of the fluids, the physical properties of the heat exchanger materials, the configuration and area of the heat transfer surfaces, the extent of scale or deposits on the heat transfer surfaces, and the ambient conditions.University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 2/9 Theory Heat transfer by conduction and convection between two fluids separated by a cylindrical tube (Figure 2) is described by Q. QAUT Thc() (1) Further manipulations can be made to find that: 11 1 1 121UA U A U A h ARALkrrRAhAiioo oofoooifiiii   ,,ln (2) Where h is the heat transfer coefficient, U is the overall heat transfer coefficient, and Rf is the fouling factor. The fouling factor is merely a term used to account for the additional thermal resistance caused by rust formation, scale buildup, etc. For fully developed, turbulent flow in tubes where the Reynolds number is between 2300 and 5106 and the Prandtl number is between 0.5 and 2000, an empirical correlation to determine h proposed by Gnielinski (1976) is widely used. NuhDkDHD 810001127 8 11223Re Pr.Pr (3a) where, for smooth tubes, the friction factor is given by  079 1642.lnRe .D (3b) All fluid properties are taken at the average fluid temperature. Thus, the convective coefficients are determined from equation (3a,b) and used in equation (2) to find the overall heat transfer coefficient, U. The heat transfer rate, Q, in equation (1) is not suitable for this case since the temperature difference between the hot and cold fluids varies between inlet and outlet. The equation recommended for parallel-flow is given by:   QUATT T TTT T Th in c in h out c outhin cin hout cout ,, , ,,, , ,ln / (4) and for counter-flow: QUATT T TTT T Th in c out h out c inh in c out h out c in,, , ,,, , ,ln / (5)University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 3/9 Figure 2, Heat Transfer Model Heat Exchanger Effectiveness The effectiveness, , of a heat exchanger is defined as Actual heat transferredMaximum possible heat transfer Let us also define the heat capacity rate, C, for the cold and hot fluids as: Cmchhph, Cmcccpc, Chcminminimum of C and C Chcmaxmaximum of C and C Where m is the mass flow rate and cp is the specific heat at constant pressure. With these definitions, the heat exchanger experimental effectiveness can be derived as CT TCT TCT TCT Tc c out c inhin cinhhin houthin cin,,min , ,,,min , , (6) (Please note that in your experiment the RHS and LHS of equation (6) will not be equal. This is due to imperfect insulation of the heat exchanger from the surroundings. You should therefore calculate both and use the minimum of the two values in your analysis.) Further manipulation gives the following equation for theoretical effectiveness for a parallel flow, concentric tube heat exchanger  111Exp NTU CCrr (7) For a counter flow heat exchanger the equation becomes  1111Exp NTU CCExp NTU Crrr (8)University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 4/9 where NTU is the number of transfer units which is defined as NTU  UA/Cmin and Cr is the heat capacity ratio and is defined as Cr  Cmin/Cmax. Venturi Flow Meter This experiment makes use of a venturi flow meter to indicate volumetric flow rate. In this type of flow meter the pressure differential developed across a venturi will determine the actual flow rate in the system. The equation to determine the flow rate is given by: ()VBAppvw212421 (9) where the area ratio  = d2/d1 = (A2/A1)1/2. Apparatus Heat Exchanger The concentric tube heat exchanger in this experiment is operable in both the parallel- and counter-flow configuration and consists of the following: 2 Venturi flow meters 2 Mercury manometer tubes 4 Thermocouples Thermocouple Controller The physical characteristics of the heat exchanger are: Inner Tube ID 0.529 in. Inner Tube OD 0.625 in. Outer Tube ID . 0.730 in. Heat Exchanger Length 57 in. Tube Material 100% Cu Fouling Factor, Rf 0.0003 m2K/W To correct the temperature values read from the digital display, use the following calibration equation: T(F) = 1.241  T(reading) - 17.58University of Texas at Arlington MAE 3183, Measurements II Laboratory Concentric Tube Heat Exchanger 5/9 Figure 4, Heat Exchanger Flow Diagram Venturi Flow Meter The venturi flow meters used in the experiment have the following characteristics: Bv, = 0.91 d1 = 0.50 in. d2 = 0.25 in. Flow Figure 3, Venturi Flow Meter used to measure volumetric flow rateUniversity of Texas at Arlington MAE 3183, Measurements