7/30/2019 Heat Exchangers of Polypropylene
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Exchangers of polypropylene
//www.segerfrojd.com/ppvsmetal.htm[21/11/2012 2:50:49 p.m.]
How can Polypropylener ansfer h eat as good as
metals?
Many people have the opinion that heat
exchangers of plastics should have a very poor
performance because of the poor thermal
onductivity of plastics. This is not the case!
Figu re 1 . Heat transfer through a single wall.
The heat transfer through a single wall is
dependent on three factors; the overall heat
ransfer coefficient, U, the heat transferring area,
A, and the temperature difference between the
heat emitting and the heat absorbing media.
n a heat exchanger the overall heat transfer
oefficient, U, is dependent on; the convective
heat transfer coefficient for both the hot and the
old fluid and the thermal conductivity of the wall
material.
t can be shown that the overall heat transfer
oefficient, U, is governed by the individual
onvection heat transfer coefficient, h, on the gas
air) side of the walls in the heat exchanger.
This means that the heat transfer in this case is
elatively insensitive to the type of material used
n the walls. If the heat exchanger was made of
luminum or steel instead of polypropylene, the
overall heat transfer would not change
ignificantly.
The surface characteristics of the heat exchanger
s also important, affecting pressure drop and
ouling tendencies. Organic and mineral deposits
re less adherent to the plastic surface, compared
D ef i n i t i on o f t heovera l l hea t t rans fe rcoef f ic
ient , U
The overall heat transfercoefficient for a plate heatexchanger
is calculated by;
where
U [W/m2K] is the overall heattransfer coefficient
A [m2] is the contact area foreach fluid side
k [W/mK] is the thermalconductivity of the material
h [W/m2K] is the individualconvection heat transfercoefficient
for each fluid
xw [m] is the wall thickness
Therm a l res istance
The overall heat transfercoefficient can also be calculatedby
the view of thermalresistances. This means that thewall is split in
areas of thermalresistance, i.e. the heat transferbetween the fluid
and the wall isone resistance, the wall it self isalso one and
lastly the transferbetween the wall and the secondfluid is the last
thermalresistance.Surface coatings, like fouling andepoxy that is
commonly usedwith aluminium in heat
exchangers, adds extra thermalresistances decreasing theoverall
heat transfer.
The rm a lconduc t i v i t y , k
The thermal conductivity, k, forsome typical materials used
inplate heat exchangers is shownbelow.
Polypropylene: 0.12 W/mKStainless steel: 21 W/mKAluminium: 221
W/mK
Conv ect ion h eatt r ans fe r coe f f i c ien t ,h
The convection heat transfercoefficient, h, is dependent onthe
type of media, gas or liquid,the flow properties such asvelocity
and other flow andtemperature dependent
Sim p le exam p les
Consider a single wall withmedia 1 on the left side thatransfers
heat to media 2 onthe right side of the wall. Thwall thickness is
assumed to0.1mm and the material is Paluminium or stainless
steelThe overall heat transfer
coefficient, U, for a single w(with equal areas) is;
Exam p le 1Assume that media 1 and 2 air with the convection
heattransfer coefficient hAir=50
W/m2KThe overall heat transfercoefficient becomes:
PP: U=24.5 W/m2K
STEEL: U=25.0 W/m2K
ALU: U=25.0 W/m2KIf we ignore the influence ofthe wall U
becomes:
Thus with air-to-air the walmaterial is irrelevant, since
governed by hAir.
Exam p le 2Assume that media 2 is a liq(water) with the
convectionheat transfer coefficient
h2=1000 W/m2KThe overall heat transfercoefficient becomes:
PP: U=45.8 W/m2K
STEEL: U=47.6 W/m2K
ALU: U=47.6 W/m2KOnce again we see that U isgoverned by hAir,
and the
variation between differentmaterials is small.
Exam p le 3Assume that media 1 and 2 water with the convection
htransfer coefficient
hWater=1000 W/m2K
The overall heat transfercoefficient now becomes:
PP: U=353 W/m2K
STEEL: U=499 W/m2K
ALU: U=500 W/m2KIf the thermal conductivity ivery large or the
thickness othe wall is very thin, then th
