Introduction

The overall heat transfer coefficient is being used to determine the rate of heat transfer from one liquid at an average immense temperature T₁ to a second liquid at an average immense temperature T₂. This transfer happens through a solid surface (where bulk temperature T₁ > bulk temperatureT₂).

Overall Heat Transfer Coefficient

In thermodynamics and mechanics, the overall heat transfer coefficient is also known as the film coefficient or film effectiveness. The overall heat transfer coefficient is the proportional representation constant between the heat flux and the thermodynamic driving force for the particular heat flow (i.e., the temperature difference, T):

For combined modes, the overall heat transfer coefficient is reported in terms of overall conductance. This is also called the heat transfer coefficient and is represented by ‘U’. Within this particular instance, the heat transfer rate is as follows:

 Q = hA( T2– T1)

A = surface area of the heat transfer

T2 = temperature of the fluid surrounding

T1 = temperature of the solid surface in use

The overall heat transfer coefficient formula is as follows:

h= qT

Where:

q = heat flux, It is also denoted by W/m2. It means it is the total thermal power per unit area.

Hence we can also mention q = d Q/dA

h = represents the heat transfer coefficient. It can also be denoted as W/(m2•K)

T = It can also be denoted as K. It means the temperature difference between the solid surface and the fluid area corresponding to it.

This method is used to calculate the heat transfer between a fluid & a solid which is done through a convection or phase transition. SI unit for heat transfer coefficient is watts per square meter kelvin. It can be denoted as W/mK. (m2K).

Hence the overall heat transfer coefficient units is W/(m2•K).

Overall heat transfer coefficient is a way of measuring the ability to sufficiently transfer heat through a series of semiconducting and convection cooling barriers’ . It is commonly used to determine heat transfer in heating systems and heat pumps. , but it can also be used to solve other problems.

In the case of a heating boiler, the overall heat transfer coefficient can be used to compute the overall heat transfer between the two streams in the heating boiler using the following formula:

q= UATLM

Where:

q =  rate of heat transfer. Also denoted as W

U = overall heat transfer coefficient . It can also be denoted as W/(m2•K)

A = surface area’s heat transfer. Also denoted as m2

TLM = logarithmic mean temperature difference. Also denoted as K. The logarithmic mean temperature difference is being used to calculate the temperature’s driving force which facilitates the heat transfer in any particular flow systems, particularly heat inverters, heated or pipes.

Ways Of Calculating Heat Transfer Coefficients

There are numerous ways of calculating heat transfer coefficients in various heat transfer methods, liquids, velocity distribution, and thermohydraulic circumstances. It is frequently determined by dividing the convection fluid’s thermal conductivity by a length scale. 

The Nusselt number is frequently used to determine the heat transfer coefficient. This  Nusselt number is a dimensionless number. There are also online calculators dedicated to calculate the heat-transfer among fluid domains. The experimental evaluation of the heat transfer coefficient presents some difficulties, particularly when measuring small flow rates.

Conclusion 

We discussed the Overall heat transfer coefficient & overall heat transfer coefficient formula and other related topics through the study material notes on the overall heat transfer coefficient. We also discussed overall heat transfer coefficient units to give you proper knowledge. 

The overall heat transfer coefficient is being used to compute the total heat transfer that can happen through the design of a wall or reboiler or exchanger. The overall heat transfer coefficient is affected by the fluid flow and their characteristic properties on both sides of the wall, and the wall’s and transmission surface’s properties.