Plate Heat Exchangers

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Purpose of the Equipment:

Plate heat exchangers (PHE), often called plate-and-

frame heat exchangers, are used to change the

temperature of a liquid, vapor or gas media.

A thin, corrugated plate is used to transfer the heat

from the media on one side of the plate to the

media on the other side.

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The plate heat exchanger consists of a frame with end plates

which squeeze the corrugated heat transfer plates. Figure 2

shows a plate pack of corrugated plates with portholes for the

media to flow.

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Principles of How the Equipment Works:

Plate heat exchangers use the thin plates to keep two media of

different temperatures apart while allowing heat energy to

flow between them through the plate. The heat energy

transfer across the plate acts to change the temperatures of

the two media. The hotter one becomes cooler, and the

colder one becomes hotter.

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Gasketed plate heat exchangers:

Plate thickness vary from 0.5 and 3 mm.

Gap between two plates 1.5 to 5 mm.

Plate surface areas range from 0.03 to 1.5 m2, with a plate width : length

ratio from 2.0 to 3.0.

The size of plate heat exchangers can vary from 0.03 m2 to 1500 m2.

The maximum flow-rate of fluid is limited to around 2500 m3/h.

Plates are available in a wide range of metals and alloys; including

stainless steel aluminum and titanium.

A variety of gasket materials is also used.

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Advantages Plate heat exchangers are easier to maintain.

Low approach temps can be used, as low as 1 °C, compared with

5 to 10 "C for shell and tube exchangers.

Plate heat exchangers are more flexible, it is easy to add extra

plates.

Plate heat exchangers are more suitable for highly viscous materials.

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow.

Fouling tends to be significantly less in plate heat exchangers. 3/4/2014 15 Alok Garg

Disadvantages:

A plate is not a good shape to resist pressure and plate heat

exchangers are not suitable for pressures greater than about 30

bar.

The selection of a suitable gasket is critical.

The maximum operating temperature is limited to about 250°C,

due to the performance of the available gasket materials.

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PLATE HEAT EXCHANGER DESIGN

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PROCEDURE The design procedure is similar to that for shell and tube exchangers.

1. Calculate duty, the rate of heat transfer required.

2. If the specification is incomplete, determine the unknown fluid temperature

or fluid flow-rate from a heat balance.

3. Calculate the log mean temperature difference, ∆TLM.

4. Determine the log mean temperature correction factor, Ft.

5. Calculate the corrected mean temperature difference ∆TM = Ft x ∆TLM.

6. Estimate the overall heat transfer coefficient.

7. Calculate the surface area required.

8. Determine the number of plates required = total surface area/area of one

plate.

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PROCEDURE

9. Decide the flow arrangement and number of passes.

10. Calculate the film heat transfer coefficients for each stream.

11. Calculate the overall coefficient, allowing for fouling factors.

12. Compare the calculated with the assumed overall

coefficient. If satisfactory, say —0% to + 10% error, proceed.

If unsatisfactory return to step 8 and increase or decrease

the number of plates.

13. Check the pressure drop for each stream.

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FLOW ARRANGEMENTS

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Estimation of the temperature correction factor:

For PHE it is convenient to express the LMTD correction factor, Ft, as a function of the

NTU, and the flow arrangement (number of passes).

The correction will normally be higher for a PHE than for a STE operating with the same

temperatures.

For rough sizing purposes, the factor can be taken as 0.95 for series flow.

The number of transfer units is given by:

NTU = (t0-ti)/ ∆TLM

where

ti = stream inlet temperature, °C,

to = stream outlet temperature, °C,

∆TLM = log mean temperature difference, °C.

Typically, the NTU will range from 0.5 to 4.0, and for most applications will lie between

2.0 to 3.0.

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Heat Transfer Coefficient:

The corrugations on the plates will increase the projected plate

area, and reduce the effective gap between the plates.

The channel width equals the plate pitch minus the plate

thickness.

There is no heat transfer across the end plates, so the number of

effective plates will be the total number of plates less two.

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