Abstract
A
plate heat exchanger is a type of heat exchanger that uses metal plates to
transfer heat between two fluids. This has a major advantage over a
conventional heat exchanger in that the fluids are exposed to a much larger
surface area because the fluids spread out over the plates. This facilitates
the transfer of heat, and greatly increases the speed of the temperature
change. The plate heat exchanger (PHE) is a specialized design well suited to
transferring heat between medium- and low-pressure fluids. Welded, semi-welded
and brazed heat exchangers are used for heat exchange between high-pressure
fluids or where a more compact product is required. The hot fluid flows in one
direction in alternating chambers while the cold fluid flows in true countercurrent
flow in the other alternating chambers. The heat transfer surface consists of a
number of thin corrugated plates pressed out of a high grade metal. The pressed
pattern on each plate surface induces turbulence and minimizes stagnant areas
and fouling. Unlike shell and tube heat exchangers, which can be custom-built
to meet almost any capacity and operating conditions, the plates for plate and
frame heat exchangers are mass produced using expensive dies and presses. In
this paper we designed the PHE for the required operating conditions. In the
design we calculated the overall heat transfer coefficient of PHE. The heat
transfer rate and the number of plates required for the PHE were also
calculated. Cost optimization of the designed PHE was carried out and it has
been found that there is a considerable drop in the cost of the heat exchanger.
Introduction
The
plate heat exchanger consists of a pack of corrugated metal plates with
portholes for the passage of the two fluids between which heat transfer will
take place. The plate pack is assembled between a fix frame plate and a movable
pressure plate and compressed by tightening bolts. The plates are fitted with a
gasket which seals the interpolate channel and directs the fluids into
alternate channels. The number of plates is determined by the flow rate,
physical properties of the fluids, pressure drop and temperature program. The
plate corrugations promote fluid turbulence and support the plates against
differential pressure. The plate and the pressure plate are suspended from an
upper carrying bar and located by a lower guiding bar, both of which are fixed
to a support column. Connections are located in the frame plate or, if either
or both fluids make more than a single pass within the unit, in the frame and
pressure plates. The survey of the literature regarding the plate heat
exchanger and using of various compressor oils in the household refrigerator
and air-conditioners are listed. ZhenHua Jin et. al. [1] designed and estimated
the pressure drop of PHE. His investigation verified that the pressure drop in
PHE is comparatively lesser than the shell and tube heat exchanger. Aydın
Durmus et. al. [2] he investigated the heat transfer in plate heat exchanger
and he found that the heat transfer rate in plate heat exchanger is much more
than that of conventional heat exchanger
Working Principle
Channels
are formed between the plates and the corner ports are arranged so that the two
media flow through alternate channels. The heat is transferred through the
plate between the channels, and complete counter-current flow is created for
highest possible efficiency. Figure1 shows a plate heat exchanger. The
corrugation of the plates provides the passage between the plates, supports
each plate against the adjacent one and enhances the turbulence, resulting in
efficient heat transfer.
Design of Heat
Exchanger
Mass
flow rate = 231000 kg/hr
Hot
side : Slurry
Tsi
= 86.6°C
Tso
= 66°C
Cold
side:
Cooling
water from cooling tower
Tci
= 34°C.
Q
= mCp (Tsi – Tso)
=
231000 x 0.238 (86.6 – 66)
=
11.325 x 105 Kcal/hr Tco
=
Tci + [Q/ m Cp]
=
34 + [11.325x 105/ (231000x 0.238)
=
54.599°C
To find LMTD
LMTD
[(Tsi-Tco) – (Tso-Tci)]/ln
[(Tsi-Tco)/(Tso-Tci)]
=
[(86.66-54.599)-(66-34)]/ln [(86.6 - 54.599)/(66-34)]
=
32.00105 Uavg
=
1/[(1/H)+(x/k)+(1/Hc)+dFc] Eliminate [1/Hc], dFc H
=
Film coefficient Re avg
= 32682.179 H
=
0 .742 x Cp x G x (Re avg) - 0.62 x (Pr avg) - 0.667
=
0.742x 0.238 x (231000/1.02) x (32682.17) -0.62 x 1.465- 0.667
Where
Pr
= µCp/k k
= 0.573 x 1.02 kcal/hr m² ºC
= 0.58446 Pr
= 1.465
Then
H = 492.817 Kcal/ hr m² º C U
= 327.17 Kcal /
hr m² º C
Number
of Plates, N = At / Ap
Assume
Area of Plate, Ap =1.02 m²
At
= (Q / UxLMTDxF)
Where
Q = 11.325 x 10 5 Kcal/ hr
LMTD
= 32.005 U
=
327.17 Kcal/hr m² º C F = 0.98
=
(11.3258 x10 5 )/(327.17 x 32.0005x 0.98)
=
110.377 m 2 Number of plates
N = At /Ap
= 110.377/1.02
= 108.2 = 108
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