Estimating Heat Exchanger Temperatures

Chapter 1 provides a summary of important equations for estimating the terminal temperatures in a heat exchanger. Here we formalize a short estimating procedure for a countercurrent flow situation. Assume that a specifier of a heat exchanger has defined a preliminary sizing of the unit. The system requires heat and material balances. 

The only information available are the inlet temperatures of the hot and cold fluids and estimates for the overall heat transfer coefficient U and the heat transfer surface A. The flows are fixed and the specific heats of the fluids known. 

The problem is different from typical heat transfer problems. The heat balance is not straightforward because the outlet temperatures are unknown. A trial-and-error procedure is therefore required. 

Parameters Cj and Cj are specific heats of the cold and hot fluids in Btu respectively, and the log-mean temperature difference is defined as follows  

The heat transfer rate can be determined from the following equation  

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Ganapathy presents a shortcut technique for estimating heat exchanger tube wall temperature, which so often is needed in establishing the fluid film temperature at the tube wall ... 

This approach consists in estimating heat exchanged by each stream in order to determine the heat provided to the system by the reaction. Actually, at steady state, the heat of reaction wiU lead to a temperature rise of process and utility streams, while the utility stream aims at limiting this increase (cooling effect). The conversion rate, x, is easily calculated by... 

Chapter 3 of Volume 1 discusses many of the basic properties of gas and methods presented for calculating them. Chapter 6 of Volume 1 contains a brief discussion of heat transfer and an equation to estimate the heat required to change the temperature of a liquid. This chapter discusses heat transfer theory in more detail. The concepts discussed in this chapter can be used to predict more accurately the required heat duty for oil treating, as well as to size heat exchangers for oil and water. 

Comparison between the heat exchanged per unit of volume during oxidation experiment in the Shimtec reactor and the maximal heat exchanged in a classical batch reactor (with a double jacket) highlights the effectiveness of the former. Indeed, in oxidation reaction experiments, a mean value of the heat exchanged per unit of volume in the HEX reactor is estimated with utility stream temperature of 47 °C ... 

Table 3.3 Comparison of the calculated heat-transfer coefficients, heat-exchange areas and the estimated temperature gradients between the central reactor area and the outer wall of the reactor.

Though these conditions will not be strictly satisfied in practical heat exchangers, the Ft values obtained from the curves will give an estimate of the true mean temperature difference that is sufficiently accurate for most designs. Mueller (1973) discusses these... 

The Geothermal Response Test as developed by us and others has proven important to obtain accurate information on ground thermal properties for Borehole Heat Exchanger design. In addition to the classical line source approach used for the analysis of the response data, parameter estimation techniques employing a numerical model to calculate the temperature response of the borehole have been developed. The main use of these models has been to obtain estimates in the case of non-constant heat flux. Also, the parameter estimation approach allows the inclusion of additional parameters such as heat capacity or shank spacing, to be estimated as well. 

A convenient way to display cost-capacity data is by algorithms. They are readily adaptable for computerized cost estimation programs. Algorithm modifiers in equation format may be used to account for temperature, pressure, material of construction, equipment type, etc. Equation (9-2) is an example of obtaining the cost of a shell-and-tube heat exchanger by using such modifiers. 

The chemical process gives the enthalpy of reaction, the flow rate, the reaction time, and the required reaction temperature. The first step in the sizing procedure is to calculate the required number of channels for the heat exchanger. Then the pass arrangement is selected in order to achieve the highest possible Reynolds number within an acceptable pressure drop. For example, if the total number of channels is fixed by the residence time channels in series will induce high velocities and high pressure drop channels in parallel will induce low velocities and low pressure drop. The second step is to estimate the heat transfer coefficient and to check that the heat flux can effectively be controlled by the secondary fluid (the lower heat transfer coefficient should be on the reaction side). 

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