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Heat Transfer/Heat Exchangers Blogs

Here you will find informational articles on topics related to the Excel spreadsheets for civil and mechanical engineering calculations available from the DOWNLOADS page.  This includes articles in the clickable categories below: pipe flow calculations, open channel flow, heat transfer/heat exchangers, storm water/hydrology, continuous beam analysis and design, open channel flow measurement, and pipe flow measurement topics.  Scroll down on each category page to see all of the articles.

Similar blog articles are available at our companion site, www.EngineeringExcelSpreadsheets.com.

Pipe Flow CalculationsOpen Channel FlowHeat Transfer/Heat ExchangersStorm Water/HydrologyCurrentWastewater TreatmentOpen Channel Flow MeasurementStructural Analysis and Design of BeamsPipe Flow MeasurementEngineering EconomicsFluid PropertiesSurveying

Thermal Design of Double Pipe Heat Exchanger

Posted on Sunday, December 21, 2014 at 11:39 PM

 

Where to Find a Spreadsheet for Thermal Design of a Double Pipe Heat Exchanger

For an Excel spreadsheet for thermal design of a double pipe heat exchanger, click here to visit our spreadsheet store.  Read on for information about the use of a spreadsheet for thermal design of a double pipe heat exchanger.

The Basic Equation for Thermal Design of a Double Pipe Heat Exchanger

The basic heat exchanger design equation is:  Q = U A ΔTlm,    where:

  • Q = the rate of heat transfer between the two fluids in the heat exchanger in But/hr (kJ/hr for S.I. units)
  • U is the overall heat transfer coefficient in Btu/hr-ft2-oF  (kJ/hr-m2-K for S.I. units)
  • A is the heat transfer surface area in ft2 (m2 for S.I. units)
  • ΔTlm is the log mean temperature difference in oF,  (K for S.I units)  calculated from the inlet and outlet temperatures of both fluids.

In an Excel spreadsheet for thermal design of a double pipe heat exchanger, the heat exchanger equation can be used to calculate the required heat exchanger area for known or estimated values of the other three parameters, Q, U, and ΔTlm.  Each of those parameters will be discussed briefly in the next three sections.

The Log Mean Temperature Difference, ΔTlm , for Thermal Design of a Double Pipe Heat Exchanger

Equation for heat exchanger thermal design calculations spreadsheetThe driving force for a heat transfer process is always a temperature difference. For heat exchangers, there are always two fluids involved, and the temperatures of both are changing as they pass through the heat exchanger.  Thus some type of average temperature difference is needed.  Many heat transfer textbooks (e.g. ref #1 below) show double pipe heat exchanger diagram for heat exchanger thermal design calculations spreadsheetthat the log mean temperature difference is the appropriate average temperature difference to use for heat exchanger design calculations.  The definition of the log mean temperature difference is shown in the figure above.  The meanings of the four temperatures in the log mean temperature difference equation are rather self explanatory as shown in the diagram of a counterflow double pipe heat exchanger at the right.

The Heat Transfer Rate, Q, for Thermal Design of a Double Pipe Heat Exchanger

In order to use the heat exchanger design equation to calculate a required heat transfer area,  a value is needed for the heat transfer rate, Q.  This rate of heat flow can be calculated if the flow rate of one of the fluids is known along with its specific heat and the required temperature change for that fluid. The equation to be used is shown below for both the hot fluid and the cold fluid:

Q = mH CpH (THin - THout) = mC CpC (TCout - TCin), where

  • mH is the mass flow rate of the hot fluid in slugs/hr (kg/hr for S.I. units).
  • CpH is the specific heat of the hot fluid in Btu/slug-oF (kJ/kg-K for S.I. units).
  • mC is the mass flow rate of cold fluid in slugs/hr (kg/hr for S.I. units).
  • CpC is the specific heat of the cold fluid in Btu/slug-oF (kJ/kg-K for S.I. units).
  • The temperatures (THin, THout, TCout, & TCin) are the hot and cold fluid temperatures going in and out of the heat exchanger, as shown in the diagram above.  They should be in oF for U.S. or K for S.I. units.

The heat transfer rate, Q, can be calculated in a preliminary heat exchanger design spreadsheet if the flow rate, heat capacity and temperature change are known for either the hot fluid or the cold fluid. Then one unknown parameter can be calculated for the other fluid.  (e.g. the flow rate, the inlet temperature, or the outlet temperature.)

The Overall Heat Transfer Coefficient, U, for Thermal Design of a Double Pipe Heat Exchanger

The overall heat transfer coefficient, U, depends on the convection coefficient inside the pipe or tube, the convection coefficient on the outside of the pipe or tube, and the thermal conductivity of the pipe wall.  See the article, Forced Convection Heat Transfer Coefficient Calculations, for information about calculating the heat transfer coefficients and click here to visit our spreadsheet store, for spreadsheets to calculate the inside and outside convection coefficients and to calculate the overall heat transfer coefficient.

An Excel Spreadsheet for Thermal Design of a Double Pipe Heat Exchanger

The screenshot below shows a screenshot of an Excel spreadsheet for hthermal design of a double pipe heat exchanger.  The image shows only the beginning of the calculations.  The rest of the spreadsheet will calculate the length of pipe needed, the length of each pass for a selected number of 180 degree bends, and the pressure drop through the inside of the pipe.  Why bother to make these calculations by hand?  This Excel spreadsheet is available in either U.S. or S.I. units at a very low cost at in our spreadsheet store.

Heat Exchanger Thermal Design Calculations Spreadsheet

References

1. Kuppan, T., Heat Exchanger Design Handbook, CRC Press, 2000.

2. Kakac, S. and Liu, H., Heat Exchangers: Selection, Rating and Thermal Design, CRC Press, 2002.

3. Bengtson, H., Fundamentals of Heat Exchangers, an online, continuing education course for PDH credit.

4. Bengtson, H., Heat Exchanger Thermal Design Calculations Spreadsheet, an online blog article

 

List of Blog Articles - Heat Transfer/Heat Exchangers

Posted on Friday, December 28, 2012 at 1:48 PM

Scroll down for the following blog articles in this category:

 

  • Calculation of Natural Convection Heat Transfer Coefficients
  • Forced Convection Heat Transfer Coefficients

Excel Spreadsheets for Preliminary Heat Exchanger Design

Posted on Thursday, August 11, 2011 at 12:20 PM

 Introduction

If you want to obtain an Excel spreadsheet for preliminary design of double pipe and/or shell and tube heat exchangers, click here to visit our download page.  Read on for information about the use of an Excel spreadsheet for preliminary heat exchanger design calculations.

The Heat Exchanger Design Equation

The basic heat exchanger design equation is:  Q = U A ΔTlm

where:

  • Q = the rate of heat transfer between the two fluids in the heat exchanger in But/hr (kJ/hr for S.I. units)
  • U is the overall heat transfer coefficient in Btu/hr-ft2-oF  (kJ/hr-m2-K for S.I. units)
  • A is the heat transfer surface area in ft2 (m2 for S.I. units)
  • ΔTlm is the log mean temperature difference in oF,  (K for S.I units)  calculated from the inlet and outlet temperatures of both fluids.

For design of heat exchangers, the basic heat exchanger design equation can be used to calculate the required heat exchanger area for known or estimated values of the other three parameters, Q, U, and ΔTlm.  Each of those parameters will be discussed briefly in the next three sections.

The Log Mean Temperature Difference, ΔTlm

The driving force for a heat transfer process is always a temperature difference. For heat exchangers, there are always two fluids involved, and the temperatures of both are changing as they pass through the heat exchanger.  Thus some type of average temperature difference is needed.  Many heat transfer textbooks (e.g. ref #1 below) showthat the log mean temperature difference is the appropriate average temperature difference to use for heat exchanger calculations.  The definition of the log mean temperature difference is shown in the figure at the left.  The meanings of the four temperatures in the log mean temperature difference equation are rather self explanatory as shown in the diagram of a counterflow double pipe heat exchanger at the right.

The Heat Transfer Rate, Q

In order to use the heat exchanger design equation to calculate a required heat transfer area,  a value is needed for the heat transfer rate, Q.  This rate of heat flow can be calculated if the flow rate of one of the fluids is known along with its specific heat and the required temperature change for that fluid. The equation to be used is shown below for both the hot fluid and the cold fluid:

Q = mH CpH (THin - THout) = mC CpC (TCout - TCin), where

  • mH is the mass flow rate of the hot fluid in slugs/hr (kg/hr for S.I. units).
  • CpH is the specific heat of the hot fluid in Btu/slug-oF (kJ/kg-K for S.I. units).
  • mC is the mass flow rate of cold fluid in slugs/hr (kg/hr for S.I. units).
  • CpC is the specific heat of the cold fluid in Btu/slug-oF (kJ/kg-K for S.I. units).
  • The temperatures (THin, THout, TCout, & TCin) are the hot and cold fluid temperatures going in and out of the heat exchanger, as shown in the diagram above.  They should be in oF for U.S. or K for S.I. units.

The heat transfer rate, Q, can be calculated if the flow rate, heat capacity and temperature change are known for either the hot fluid or the cold fluid. Then one unknown parameter can be calculated for the other fluid.  (e.g. the flow rate, the inlet temperature, or the outlet temperature.)

The Overall Heat Transfer Coefficient, U

The overall heat transfer coefficient, U, depends on the convection coefficient inside the pipe or tube, the convection coefficient on the outside of the pipe or tube, and the thermal conductivity of the pipe wall.  See the article, Forced Convection Heat Transfer Coefficient Calculations, for information about calculating the heat transfer coefficients and click here to visit our download page, for spreadsheets to calculate the inside and outside convection coefficients and to calculate the overall heat transfer coefficient.

An Excel Spreadsheet as a Preliminary Heat Exchange Design Calculator

The Excel spreadsheet template shown below can be used to carry out preliminary design of a double pipe heat exchanger.  The image shown only the beginning of the calculations.  The rest of the spreadsheet will calculate the length of pipe needed, the length of each pass for a selected number of 180 degree bends, and the pressure drop through the inside of the pipe.  Why bother to make these calculations by hand?  This Excel spreadsheet and others with similar calculations for a shell and tube heat exchanger are available in either U.S. or S.I. units at a very low cost at www.engineeringexceltemplates.com or in our spreadsheet store.

References

1. Kuppan, T., Heat Exchanger Design Handbook, CRC Press, 2000.

2. Kakac, S. and Liu, H., Heat Exchangers: Selection, Rating and Thermal Design, CRC Press, 2002.

3. Bengtson, H., Fundamentals of Heat Exchangers, an online, continuing education course for PDH credit.

Calculation of Natural Convection Heat Transfer Coefficients

Posted on Saturday, July 23, 2011 at 4:48 PM

 Introduction

If you want to obtain an Excel spreadsheet for natural convection heat transfer coefficient calculations, click here to visit our download page.  Read on for information about natural convection heat transfer coefficients and Excel spreadsheets to obtain a value for them.

Convection heat transfer takes place between a solid surface and fluid that is at a different temperature and is in contact with the surface.  If the fluid is flowing past the surface due to an external driving force like a fan or pump, then the heat transfer is called forced convection.  When  fluid motion is due to density differences within the fluid (caused by temperature variation), then the heat transfer is called natural convection or free convection.

Newton's Law of Cooling for Natural Convection Heat Transfer

Newton's Law of Cooling [ Q = hA(Ts - Tf) ] is a simple expression used for the rate for convective heat transfer with either forced or natural convection.  The parameters in Newton's Law of Cooling are:

  • Q, the rate of forced convection heat transfer (Btu/hr - U.S. or W - S.I.)
  • Ts, the solid temperature (oF - U.S. or oC - S.I.)
  • Tf, the fluid temperature (oF - U.S. or oC - S.I.)
  • A, the area of the surface that is in contact with the fluid (ft2 - U.S. or m2 - S.I.)
  • h, the convective heat transfer coefficient (Btu/hr-ft2-oF - U.S. or W/m2-K - S.I.)

Dimensionless Nusselt, Rayleigh, Grashof, and Prandtl Numbers

Natural convection heat transfer coefficients typically are estimated using correlations of dimensionless numbers, specifically correlations of Nusselt number (Nu) with Prandtl number (Pr), Grashof number (Gr), and/or Rayleigh number (Ra), where Ra = GrPr.  The Nusselt, Grashof and Prandtl numbers are defined in the box at the left.

Following is a list of the parameters that appear in these dimensionless numbers, with units are given for both the U.S engineering system and S.I. system of units:

  • D, a characteristic length parameter (e.g. diameter for natural convection from a circular cylinder or a sphere or height of a vertical plate)  (ft for U.S.,  m for S.I.)
  • ρ, the density of the fluid  (slugs/ft3 for U.S.,  Kg/m3 for S.I.)
  • μ, the viscosity of the fluid  (lb-sec/ft2 for U.S.,  N-s/m2 for S.I.)
  • k, the thermal conductivity of the fluid  (Btu/hr-ft-oF for U.S.,  W/m-K for S.I.)
  • Cp, the heat capacity of the fluid  (Btu/lb-oF for U.S.,  J/kg-K for S.I.)
  • g, the acceleration due to gravity (32.17 ft/sec2 for U.S.,  9.81 m/s2 for S.I.)
  • β, the coefficient of volume expansion of the fluid  ( oR for U.S.,  K for S.I.)
  • ΔT, the temperature difference between the solid surface and the fluid  ( oF for U.S., oC or K for S.I.)

The following sections provide equations for estimating the heat transfer coefficient for several common natural convection configurations.

Natural Convection from a Vertical Plane

The box at the right shows two correlations for convection heat transfer between a vertical plane and a fluid of different temperature in contact with it.  The first can be used for all values of Rayleigh number and the second is only for laminar flow, indicated by Ra < 109.  The screenshot image below shows an example of an Excel spreadsheet to calculate the natural convection heat transfer coefficient for a vertical plate using the two equations shown here.

For low cost, easy to use Excel spreadsheet packages for calculating convection heat transfer coefficients for natural convection from a vertical plane, a horizontal plane, an inclined plane, a horizontal cylinder or a sphere in either U.S. or S.I. units, click here to visit our download page.

References

1. Incropera, F.P., DeWitt, D.P, Bergman, T.L., & Lavine, A.S., Fundamentals of Heat and Mass Transfer, 6th Ed., Hoboken, NJ, John Wiley & Sons, (2007).

2. Lienhard, J.H, IV and Lienhard, J.H. V, A Heat Transfer Textbook: A Free Electronic Textbook

3. Bengtson, Harlan H, Fundamentals of Heat Transfer, an online continuing education course for engineering PDH credit

Forced Convection Heat Transfer Coefficients

Posted on Monday, August 4, 2008 at 1:41 AM

Excel templates work well for calculation of forced convection heat transfer coefficients, typically based on correlations of Nusselt number in terms of Reynolds number and Prandtl number.  Forced convection occurs when a fluid moving past a solid surface with the fluid and the solid being at different temperatures.  Newton's Law of Cooling is a simple expression for the rate for convective heat transfer:  Q = hA(Ts - Tf), where the parameters are:

  • Q = rate of forced convection heat transfer (Btu/hr - U.S. or W - S.I.)
  • Ts = the solid temperature (oF - U.S. or oC - S.I.)
  • Tf = the fluid temperature (oF - U.S. or oC - S.I.)
  • A = the area of the surface that is in contact with the fluid (ft2 - U.S. or m2 - S.I.)
  • h is the convective heat transfer coefficient (Btu/hr-ft2-oF - U.S. or W/m2-K - S.I.)

The most difficult part of forced convection heat transfer calculations is typically determination of a good value for the heat transfer coefficient, h.  The most common way of determining the heat transfer coefficient for a particular forced convection application is through a correlation for Nusselt number (Nu) in terms of Reynolds number (Re) and Prandtl number (Pr).  The definitions of these three dimensionless numbers are shown in the box at the right, where:

  • D = characteristic length (diameter for pipe flow)
  • V = characteristic fluid velocity
  • k = thermal conductivity of the fluid
  • rho =  density of the fluid
  • mu = viscosity of the fluid

Excel spreadsheets to calculate forced convection heat transfer coefficients for several common physical configurations are available at our downloads page.

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