Introduction and brief overview of Overall heat transfer Coeffcient

1. ATMIYA INSTITUTE OF TECHNOLOGY &
SCIENCE
MECHANICAL DEPARTMENT
5th SEMESTER
OVERALL HEAT TRANSFER
COEFFICIENT
(HEAT TRANSFER– 2151909)
PREPARED BY:
DHEIRYA JOSHI

2. INTRODUCTION
• The overall heat transfer coefficient is a measure of the
overall ability of a series of conductive and convective
barriers to transfer heat. It is commonly applied to the
calculation of heat transfer in heat exchangers, but can be
applied equally well to other problems.
• The overall heat transfer coefficient, or U-value, refers to
how well heat is conducted over a series of mediums. Its
units are the W/(m2°C) [Btu/(hr-ft2°F)].
• The overall heat transfer coefficient is influenced by the
thickness and thermal conductivity of the mediums
through which heat is transferred. The larger the
coefficient, the easier heat is transferred from its source to
the product being heated.

3. • In a heat exchanger, the relationship between the overall heat
transfer coefficient (U) and the heat transfer rate (Q) can be
demonstrated by t
• he following equation:
where
Q = heat transfer rate, W=J/s [btu/hr]
A = heat transfer surface area, m2 [ft2]
U = overall heat transfer coefficient, W/(m2°C) [Btu/(hr-
ft2°F)]
ΔTLM = logarithmic mean temperature difference, °C [°F]

4. To simplify the calculation, the following values may be
used as a reference for the convective heat transfer
coefficients:
Fluid Convective heat transfer coefficient (h)
Water about 1000 W/(m2°C) [176 Btu/(hr-ft2°F)]
Hot Water
1000 – 6000 W/(m2)°C [176 - 1057 Btu/(hr-
ft2°F)]
Steam
6000 – 15000 W/(m2°C) [1057 - 2641
Btu/(hr-ft2°F)]
• The heat transfer coefficient has SI units in watts per
squared meter kelvin: W/(m2K).
• Heat transfer coefficient is the inverse of thermal
insulance. This is used for building materials (R-value) and
for clothing insulation.

5. • A single plate exchanger with media A transfers heat to
media B. The wall thickness is 0.1 mm and the material is
polypropylene PP, aluminum or stainless steel.
• Media A and B are air with a convection heat transfer
coefficient of hair = 50 W/m2K.
• The overall heat transfer coefficient U per unit area can
be expressed as:
U = 1 / (1 / hA + dxw / k + 1 / hB)
• Using the values from above the overall heat transfer
coefficient can be calculated to:
Polypropylene PP : U = 24.5 W/m2K
Steel : U = 25.0 W/m2K
Aluminum : U = 25.0 W/m2K
1 W/(m2 K) = 0.85984 kcal/(h m2 oC) = 0.1761 Btu/(ft2 h oF)
**EXAMPLE**

6. THANK YOU