M4 -Group 9Teoh Jie Shun

Dominic CheongJohnny Yeung

The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material.

Formula for rate of convective heat transfer:

Variables: A - surface area of heat transfer. Ts - surface temperature Tb - temperature of the fluid at bulk temperature. h - constant heat transfer coefficient

Turbulent flows have a higher coefficient than laminar flows, due to turbulent flows having a thinner stagnant fluid film layer on heat transfer surface.

q = hA(Ts − Tb)

Thermal conductivity is material’s ability to transmit heat, measured in: > Wm/m2k = W/mk

Surface coefficient from a surface to the surrounding air or fluid Measured in w/m2k

Thermal circuits The representation of the resistance to heat flow

as though it were an electric resistor The heat energy transferred = current +thermal

resistance = electric resistor Thermal resistance : temperature difference

across a structure when a unit of heat energy flows through it in unit time

Based on the formula

A low thermal resistance should be used in the heat exchanger

More heat energy transferred Less time to heat the crude oil.

r

TTQ fi )(

Add insulating materials

The smaller the k value, the larger the corresponding thermal resistance value.

Increasing width of insulation decreases rate of heat energy transfer.

Take into account that the driving temperature difference between the two fluids varies with position.

Ways to counter it log mean temperature difference (LMTD) used as

average temperature Number of transfer units (NTU)

logarithmic average of the temperature difference between the hot and cold streams at each end of the exchanger

Given as:Countercurrent flow

(opposite direction of 2 flows):Parallel flow:

(same direction of 2 flows)

T1 = Hot Stream Inlet Temp.T2 = Hot Stream Outlet Temp.t1 = Cold Stream Inlet Temp.t2 = Cold Stream Outlet Temp.The larger the LMTD, the more heat energy is transferred.Hence we need to use a higher LMTD in our heat exchanger.

To determine the effectiveness of a heat exchanger, Find out maximum possible transfer of heat

energy

If Cr≠0, then

If If Cr=0, then

max

min

C

CCr

http://en.wikipedia.org/wiki/Heat_transfer#One_dimensional_Application.2C_Using_Thermal_Circuits

http://en.wikipedia.org/wiki/NTU_method http://en.wikipedia.org/wiki/

Log_mean_temperature_difference http://en.wikipedia.org/wiki/Heat_transfer_coefficient http://books.google.com.sg/books?id=h-

DRjBCI08QC&pg=PP106&lpg=PP106&dq=%22rate+of+heat+transfer%22+watts+joules&source=bl&ots=5g0U6i3-hB&sig=o3qP4zva_3QRK2gyMOgauPl1X3s&hl=en&ei=CaYCSvPRKo-GkAXLk-HYBA&sa=X&oi=book_result&ct=result&resnum=4#PPP106