Heat Transfer/Heat Exchangers Blogs

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 ΔT_lm,    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)
• ΔT_lm 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 ΔT_lm.  Each of those parameters will be discussed briefly in the next three sections.

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

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) show that 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 = m_H C_pH (T_Hin - T_Hout) = m_C C_pC (T_Cout - T_Cin), where

• m_H is the mass flow rate of the hot fluid in slugs/hr (kg/hr for S.I. units).
• C_pH is the specific heat of the hot fluid in Btu/slug-^oF (kJ/kg-K for S.I. units).
• m_C is the mass flow rate of cold fluid in slugs/hr (kg/hr for S.I. units).
• C_pC is the specific heat of the cold fluid in Btu/slug-^oF (kJ/kg-K for S.I. units).
• The temperatures (T_Hin, T_Hout, T_Cout, & T_Cin) 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.

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

Scroll down for the following blog articles in this category:

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

 Introduction

 Introduction

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.