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Heat Exchanger Analysis - Mechan

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					Experiment3   1   INME 4032
Table of Contents

Principle                                                                     3

Objective                                                                     3

Go a ls                                                                       3

Background                                                                    4

            a) Experimental approach                                          4

                     Overall efficiency                                      4

                     Temperature efficiencies                                5

                     Overall heat transfer coefficient U                     6

            b) Analytical approach                                            7

Experimental Setup                                                            9

            a) Tubular Heat Exchanger                                        10

                     Description of the Tubular Heat Exchanger              10

                     Technical Data                                         11

            b) Plate Heat Exchanger                                          11

                     Description of the Plate Heat Exchanger                11

                     Technical Data                                         12

            c) Shell & tube heat exchanger                                   12

                     Description of the Shell & Tube Heat Exchanger         12

                     Technical Data                                         13

Procedure                                                                    14

Discussion                                                                   14




Experiment3                                2                           INME 4032
                                          University of Puerto Rico
                                            Mayagüez Campus
                                    Department of Mechanical Engineering
                                     INME 4032 - LABORATORY II
                                             Spring 2004
                                          Instructor: Guillermo Araya
                      Experiment 3: Heat Exchanger Analysis
Principle

This experiment is designed to acquire experience on heat exchangers (being the most
usually found in industrial applications: Tubular, Plate and Shell & Tube heat
exchangers) and to understand the factors and parameters affecting the heat transfer
rates.

Objective

To acquire experience on three basic heat exchangers (Tubular, Plate and Shell &
Tube) and to understand the factors and parameters affecting the rates of heat transfer.

Goals

For co-current and counter-current operation of the equipment and flow rates (hot and
cold fluids) specified by the instructor, determine:

   a) The heat lost to the surroundings.

   b) The overall efficiency.

   c) The temperature efficiency for the hot and cold fluids.

   d) The overall heat transfer coefficient U determined experimentally.

   e) The overall heat transfer coefficient U determined theoretically. Compare with the
         experimental one.




Experiment3                               3                             INME 4032
Background

The process of heat exchange between two fluids that are at different temperatures and
separated by a solid wall occurs in many engineering applications. The device used to
implement this exchange is called a heat exchanger, and specific applications may be
found in space heating and air-conditioning, power production, waste heat recovery and
chemical processing. Heat exchangers are typically classified according to flow
arrangement and type of construction. In the first classification, flow can be
countercurrent or cocurrent (also called parallel). On the other hand, according to their
configuration, heat exchangers can be labeled as tubular, plate and shell & tube heat
exchangers.

   a) Experimental approach

      Overall efficiency

      To design or predict the performance of a heat exchanger, it is essential to
      determine the heat lost to the surrounding for the analyzed configuration. We can
      define a parameter to quantify the percentage of losses or gains. Such parameter
      may readily be obtained by applying overall energy balances for hot and cold
      fluids. If Qe is the heat power emitted from hot fluid, meanwhile Qa the heat power
      absorbed by cold fluid (neglecting potential and kinetic energy changes);

                                                           
                            Q e  m h (h h ,i  h h ,o )  m h Cp h (Th ,i  Th ,o )

                                                           
                             Q a  m c (h c,i  h c,o )  m c Cp c (Tc,i  Tc,o )

      Where,

            
       m h , m c : mass flow rate of hot and cold fluid, respectively.


       h h ,i , h h ,o : inlet and outlet enthalpies of hot fluid, respectively.


Experiment3                                       4                                    INME 4032
      h c ,i , h c ,o : inlet and outlet enthalpies of cold fluid, respectively.


      Th ,i , Th ,o : inlet and outlet temperatures of hot fluid, respectively.


      Tc ,i , Tc ,o : inlet and outlet temperatures of cold fluid, respectively.


      Cp h , Cp c : specific heats of hot and cold fluid, respectively.


     Heat power lost(or gained): Q e  Q a


                                                  Qa
     Percentage of losses or gains P                   100
                                                  Qe


     If the heat exchanger is well insulated, Qe and Qa should be equal. In practice
     these differ due to heat losses or gains to/from the environment.

     The above formulas were deducted taking into account that hot fluid is rounded
     by cold fluid. If the average cold fluid temperature is above the ambient air
     temperature then heat will be lost to the surroundings resulting in P < 100%. If the
     average cold fluid temperature is below the ambient temperature, heat will be
     gained resulting P> 100%.

     Temperature efficiencies

     A useful measure of the heat exchanger performance is the temperature
     efficiency of each fluid stream. The temperature change in each fluid stream is
     compared with the maximum temperature difference between the two fluid
     streams giving a comparison with an exchanger of infinite size.




Experiment3                                   5                                    INME 4032
          Fig 1: Countercurrent and Cocurrent operation for a shell and tube heat exchanger


                                                      Th ,inlet  Th ,outlet
     Temperature efficiency for hot fluid h                                     100
                                                      Th ,inlet  Tc,inlet


                                                          Tc,outlet  Tc,inlet
     Temperature efficiency for cold fluid c                                    100
                                                          Th ,inlet  Tc,inlet


                                                h  c
     Mean temperature efficiency  m 
                                                   2

     Subscripts h and c stand for hot and cold, respectively.

     Overall heat transfer coefficient U

     Because the temperature difference between the hot and cold fluid streams
     varies along the length of the heat exchanger it is necessary to derive an average
     temperature difference (driving force) from which heat transfer calculations can



Experiment3                                6                                             INME 4032
     be performed. This average temperature difference is called the Logarithmic
     Mean Temperature Difference (LMTD) tlm.

                       t 1  t 2
      LMTD t lm 
                     ln( t 1 / t 2 )


     Where,

     t1 = T1-T4

     t2 = T2-T3

     Note: See FIG 1. to identify temperatures in cocurrent and counterflow operation.

     We can define an overall heat transfer coefficient U as:

            Qe       W 
      U            m2K 
           At lm       

     Where,

     Qe = Heat power emitted from hot fluid

     A = Heat transmission area

  b) Analytical approach

     Up to now, a methodology to evaluate the performance of a determined heat
     exchanger has been developed. Here, an analytical study will be explained in
     order to understand the initial steps of thermal and sizing design.

     Analytical methods are only approximate in order to get an idea of the heat
     exchanger size. The overall heat transfer coefficient is calculated assuming that
     is constant along all the heat exchanger and can be predicted with convection
     correlations. Nevertheless, there are many factors that affect this value, for
     instance, the influence of bubbles, corrosion, etc. Manufacturers provide manuals
     that contain information more precise regarding the heat exchangers they trade.


Experiment3                              7                            INME 4032
     Then, it is expected that the theoretical values differ from the experimental ones,
     fundamentally due to the presence of bubbles. Of course, experimental results
     are mandatory because they reflect real conditions of operation. However, for
     heat exchanger selection it is convenience to have a methodology in order to
     estimate the overall heat transfer coefficient or the size according to given
     temperature range and flow specifications.

     Before setting the equation that determines the Overall Heat Transfer Coefficient,
     let’s take some assumptions. The conduction resistance between hot and cold
     fluid could be neglected, also resistance due to fouling.

                  1
      U
           1/ h h  1/ h c


     Where,

     hh : Heat transfer coefficient of hot fluid [W/m2K]

     hc : Heat transfer coefficient of cold fluid [W/m2K]

     In order to calculate hh and hc, the appropriate correlation will be used.

     For flow in circular tubes:

     NuD : 4.36 (Laminar flow, ReD < 2300)

     Colburn equation

     NuD : 0.023 ReD4/5 Pr1/3 (Turbulent flow, ReD > 2300)

               hD
      Nu D 
                k

     D: Diameter of tube

     k: Conductivity of fluid

     If the tube is non circular, hydraulic diameter is used, instead.


Experiment3                             8                                INME 4032
               4A c
        Dh 
                P

       Where Ac and P are the cross-sectional area and the wetted perimeter,
       respectively.

Experimental setup

There are three optional small-scale heat exchangers that can be installed to illustrate
the principles and different techniques of heat transfer between fluid streams. The heat
exchangers are individually mounted on a common bench-top Heat Exchanger Service
Unit. The unit supplies hot and cold water streams to the different heat exchangers
installed on it.

The following parameters can be modified for each small-scale heat exchanger:
volumetric flow rates of hot and cold fluids, hot fluid temperature and flow arrangements
(countercurrent or cocurrent).




           Fig. 2: Heat Exchanger Service Unit with the Tubular Heat Exchanger installed.




Experiment3                                   9                                   INME 4032
  a) Tubular Heat Exchanger




          Fig. 3: Tubular Heat Exchanger


                                                        Fig. 4: Diagram of tubular heat exchanger
                                                             under countercurrent operation.




    Description of the Tubular Heat Exchanger:

    Please refer to figures 3, 4, and 5

    The tubular heat exchanger consists of two concentric (coaxial) tubes carrying the
    hot and cold fluids. The tubes are separated into two sections.




                             Fig. 5: Diagram of tubular heat exchanger
                                     under co-current operation.




Experiment3                                10                                 INME 4032
    The accessory consists of two concentric tube heat exchangers arranged in series
    in the form of a U. The hot water flows in the inner tube and cold fluid in the outer
    annulus. The equipment allows the conversion from countercurrent to co-current
    operation.

    Six temperature sensors are installed in the hot and cold fluid inlets, outlets and
    mid positions.

    Technical Data:

             Each inner tube is constructed from stainless steel tube, 9.5mm OD.

             Each outer annulus is constructed from clear acrylic tube, 12.0mm ID.

             Each heat transfer section is 330mm long giving a combined heat transfer
              area of approximately 20000mm2. Heat transfer area is equivalent to that
              of the HT33 Shell and Tube Heat Exchanger.

  b) Plate Heat Exchanger
    Description of the Plate Heat Exchanger

    The plate heat exchanger consists of a pack of plates with sealing gaskets held
    together in a frame between end plates. Hot and cold fluids flow between channels
    on alternate sides of the plates to promote heat transfer. The plate heat exchanger
    supplied is configured for multi-pass operation with passes in series.




                                                    Fig. 7: Schematic diagram of plate heat
                                                  exchanger showing countercurrent fluid flow

              Fig. 6: Plate heat exchanger

Experiment3                                  11                           INME 4032
    The plate heat exchanger consists of a pack of seven plates and gaskets arranged
    for multi-pass operation with passes in series (Pattern of holes in the plates and
    shape of the gaskets determine the direction of flow through the exchanger). Four
    temperature sensors are installed at fluid inlets and outlets.

    Hot and cold fluid connections allow connection and conversion from
    countercurrent to co-current operation.

    Technical Data:

             Number of active plates 5

             Plate overall dimensions: 75mm x 115mm

             Effective diameter: 3.0mm

             Plate thickness: 0.5mm

             Wetted perimeter: 153.0mm

             Projected heat transmission area 0.008m2 per plate

             Correction factor for LMTD: F = 0.95

             Temperatures are measured using type K thermocouples with miniature
              plug for direct connection to the electrical console on HT30X.

             Plates are manufactured from 316 stainless steel.

  c) Shell & tube heat exchanger
    Description of the Shell & Tube Heat Exchanger

    The shell and tube heat exchanger consists of a number of tubes in parallel
    enclosed in a cylindrical shell. Heat is transferred between one fluid flowing
    through the tubes and another fluid flowing through the cylindrical shell around the



Experiment3                               12                           INME 4032
    tubes. Baffles are included inside the shell to increase the velocity of the fluid to
    improve the heat transfer.

    The exchanger is designed to demonstrate liquid to liquid heat transfer in a 1-7
    shell and tube heat exchanger (one shell and 7 tubes with two transverse baffles in
    the shell).

    Technical Data:

        Hot fluid flows in the inner tubes and cold fluid in outer shell. The seven tubes
         are constructed from stainless steel tube, 6.35mm OD. The outer annulus,
         end caps and baffles constructed from clear acrylic. The length of tube
         bundles 144mm (actual length of heat transfer region) giving nominal
         combined heat transfer area of 20 000mm2.

        Heat transfer area is equivalent to that of the Concentric Tube Heat
         Exchanger for direct comparison.

        Cold fluid enters one end of the shell at the bottom and exits at the opposite
         end at the top having flowed over and under two transverse baffles inside the
         shell.

        Temperatures are measured using type K thermocouples.

        Thermocouples are installed at the following 4 locations (when operated
         countercurrent):

         ▪ Hot fluid inlet (T1)

         ▪ Hot fluid outlet (T2)

         ▪ Cold fluid inlet (T3)

         ▪ Cold fluid outlet (T4)




Experiment3                            13                              INME 4032
     Fig. 8: Tube and shell heat exchanger.
                                                   Fig. 9: Schematic diagram of tube and shell heat
                                                     exchanger showing countercurrent fluid flow


     Hot and cold fluid connections allow connection and conversion from
     countercurrent to co-current operation.

     The stainless steel tubes can be removed from the heat exchanger for cleaning.

Procedure

Allow the system to reach steady state, and take readings and make adjustments as
instructed in the individual procedures for each experiment. Record temperatures, V, I,
and others if any. Repeat the lectures three times to assure that the system has reached
steady state.

Compute the mean value and standard deviation. Report your results for a confidence
level of 95%.

Discussion




Experiment3                                   14                                INME 4032