Apr-2010

Estimation of heat losses from process piping and equipment

An accessible predictive tool calculates surface heat losses from refinery piping and equipment. The most important part of the energy management strategy in any process industry is energy saving.

Alireza Bahadori and Hari B Vuthaluru
Curtin University of Technology

Article Summary

In this article, an attempt has been made to formulate a predictive tool that is easier to apply than existing approaches, less complicated with fewer computations, and suitable for refinery process engineers, for the rapid estimation of heat losses in terms of wind velocity and the temperature differences between process piping and equipment surfaces and the surrounding air.

The tool developed in this study could be of immense practical value for engineers and scientists to make a quick check of heat losses to air in contact with walls or surfaces without the need for experimental measurement. The results can be used in follow-up calculations to determine heat losses from process piping and equipment surfaces under various conditions. In particular, engineers should find the tool to be user-friendly with transparent calculations involving no complex expressions.

Due to limited energy resources, and environmental pollution arising from the use of fuels, energy saving has become mandatory.1 In particular, industrial and chemical processing plants contain intricate and costly piping configurations. Piping systems are also employed in many other situations, including water supply, fire protection and district cooling/heating applications.²

Several rigorous studies have been reported in the literature on the combined effects of convection and surface radiation. However, there is no simple-to-use predictive tool for an accurate estimation of combination convection and radiation film coefficients for air in contact with vertical walls or surfaces to give the combined heat transfer coefficient — in terms of the wind velocity and the temperature difference between the process piping and equipment surfaces and the surrounding air — for heat loss calculations from various cases.

In view of this shortfall, our efforts have been directed at formulating a simple-to-use, predictive tool that can serve practising engineers and applied researchers. The principal value of the proposed tool lies in its accuracy and simplicity, wherein the relevant coefficients can be retuned quickly if more data are available in the future. The case study presented here demonstrates the usefulness of the proposed tool. The present study discusses the formulation of a simple correlation that can be of significant importance for engineers.

Development of a simple predictive tool
Equation 1 calculates a coefficient, , which is the difference in temperature between a surface and the surrounding air, °C:

ΔT = Ts -Ta     (1)

The data required to develop the first correlation include reliable data3 for various values of wind velocity, and the temperature difference between the surface and the surrounding air. The following methodology has been applied to develop the predictive tool.^1,2

First, combination convection and radiation film coefficients for air in contact with vertical walls or surfaces (hcr) in W/(m.°C) are correlated as a function of the temperature difference between the surface and the surrounding air values (ΔT) in °C for different wind velocity values (v) in metres per second. Then, the calculated coefficients for these equations are correlated as a function of wind velocity values. The derived equations are applied to calculate new coefficients for equation 2 to predict combination convection and radiation film coefficients for air in contact with vertical walls or surfaces. Table 1 shows the tuned coefficients for Equations 3 to 6 according to the data.³

In brief, the following steps are repeated to tune the coefficients of Equations 1 and 2:
• Correlate the combination convection and radiation film coefficients for air in contact with vertical walls or surfaces as a function of the temperature difference between the surface and the surrounding air for a given wind
• Repeat step 1 for other values of wind velocity
• Correlate corresponding polynomial coefficients, which are obtained in the previous steps, against wind velocity, so that we have: a = f(v), 
b = f(v), c = f(v), d = f(v) (see Equations 3 to 6).

Equation 2 presents a new correlation in which four coefficients are used to correlate the combination convection and radiation film coefficients for air in contact with process piping and equipment surfaces, and the temperature difference between the surface and the surrounding air values:

hcr = a + b(ΔT) + c(ΔT)2 + d(ΔT)3    (2)

Where:
a = A1 + B1v + C1v2 + D1v3     (3)
b = A2 + B2v + C2v2 + D2v3    (4)
c = A3 + B3v + C3v2 + D3v3     (5)
d = A4 + B4v + C4v2 + D4v3     (6)
 
The tuned coefficients used in Equations 3 to 6 are given in Table 1 and help to cover the reported data with wind velocity variations up to 20 m/s and temperature gradients (the temperature of a surface less the temperature of surrounding air) up to 280°C.
Equation 7 calculates heat losses from equipment surfaces occur primarily by radiation and convection:

Q = hcr (Ao) (Ts - Ta)                                         (7)

Results
Figure 1 illustrates the results of a proposed correlation for predicting combination convection and radiation film coefficients for air in contact with walls or surfaces in comparison with some typical data obtained from the literature.³