Published: January 8, 2008

A Quick Look at the Basics

Numerous articles have been published regarding the advantages of compact heat exchangers.  Briefly, their higher heat transfer coefficients, compact size, ease of service, cost effectiveness, and their unique ability to handle fouling fluids make compact exchangers a good choice for many services.

Plate heat exchangers consist of pressed, corrugated metal plates fitted between a thick, carbon steel frame.  Each plate flow channel is sealed with a gasket, a weld, or an alternating combination of the two.  It is not uncommon for plate and frame heat exchangers to have overall heat transfer coefficient that are 3-4 times those found in shell and tube heat exchangers.

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Specifying Plate and Frame Heat Exchangers

Engineers often fail to realize the differences between heat transfer technologies when preparing a specification.  This specification is then sent to vendors of different types of heat exchangers.  Consider the following example:

A process stream requires C276 material to guard against corrosion.  The stream needs to be cooled with cooling water before being sent to storage.  The metallurgy makes the process stream an immediate candidate for the tubeside of a shell and tube heat exchanger.   The cooling water is available at 80 ⁰F and must be returned at a temperature no higher than 115 ⁰F.  The process engineer realizes that with the water flow being placed on the shellside, larger flowrates will enhance the heat transfer coefficient.  The basis for the heat exchanger quotation was specified as follows:

According to the engineer’s calculations, these basic parameters should provide a good shell and tube design with a minimum amount of C276 material (an expensive alloy).  The completed specification sheet is forwarded to many manufacturers, including those that could easily quote plate and frame or another compact technology.  A typical plate and frame unit designed to meet this specification would have about 650 ft² of area compared to about 420 ft² for a shell and tube exchanger.  A plate and frame unit designed to the above specification is limited by the allowable pressure drop on the cooling water.  If the cooling water flow is reduced to 655 GPM and the outlet water temperature allowed to rise to 115 ^°F, the plate and frame heat exchanger would contain about 185 ft² of area.  The unit is smaller, less expensive, and uses less water.  The load being transferred to the cooling tower is the same. 

The theory that applied to the shell and tube heat exchanger (increasing water flow will minimize heat transfer area), works in exactly the opposite direction for compact technologies.  The larger water flow actually drives the cost of the unit upward.  Rather than supplying a rigid specification to all heat exchanger manufacturers, the engineer should have explained his goal in regards to the process stream.  Then he could have stated the following:

“The process stream is to be cooled with cooling water.  Up to 2000 GPM of water is available at 80 ⁰F.  The maximum return temperature is 115 ⁰F.”

This simple statement could result in vastly different configurations when compared with the designs that would result from the original specification.

Design Charts for Plate and Frame Exchangers (Download Printable Copies in MS Excel)

Often, in compact heat transfer technology, engineers find themselves at the mercy of the manufacturers of the equipment.  For example, limited literature correlations are available to help in the preliminary design of plate and frame heat exchangers.  We will introduce a series of charts that can be used for performing preliminary sizing of plate and frame exchangers.  After introducing the charts, we will follow with examples to help clarify the use of the charts.  The following should be noted regarding the use of the charts:

1.     These charts are valid for single pass units with 0.50 mm thick plates.  The accuracy of the charts will not be compromised for most materials of construction.

2.     Wetted material thermal conductivity is taken as 8.67 Btu/h ft ⁰F (value for SS)

3.     Heat transfer correlations are valid for single phase, liquid-liquid designs

4.     The following physical properties were used for the basis:

5.     Degree of accuracy should be within ± 15% of the service value for the overall heat transfer coefficient, assuming a nominal 10% excess heat transfer area.

6.     For fluids with viscosities between 100 and 500 cP, used the 100 cP line of the graphs.  For fluids in excess of 500 cP, consult with manufacturers.

Figure 2: Heat Transfer Data for 0.25 < NTU < 2.0 for Plate and Frame Heat Exchangers, Water Based Properties

Figure 3: Heat Transfer Data for 2.0 < NTU < 4.0 for Plate and Frame Heat Exchangers, Water Based Properties

Figure 4: Heat Transfer Data for 4.0 < NTU < 5.0 for Plate and Frame Heat Exchangers, Water Based Properties

Figure 5: Heat Transfer Data for 0.25 < NTU < 2.0 for Plate and Frame Heat Exchangers, Hydrocarbon Based Properties

Figure 6: Heat Transfer Data for 2.0 < NTU < 4.0 for Plate and Frame Heat Exchangers, Hydrocarbon Based Properties

Figure 7: Heat Transfer Data for 4.0 < NTU < 5.0 for Plate and Frame Heat Exchangers, Hydrocarbon Based Properties

Consider the following example:
	150,000 lb/h of water is being cooled from 200 °F to 175 °F by 75,000 lb/h of SAE 30 oil.   The oil enters the exchanger at 60 °F and leaves at 168 °F.  The average viscosity of the water passing through the unit is 0.33 cP and the average viscosity of the oil in the unit is 215 cP.  The maximum allowable pressure drop through the plate heat exchanger is 15 psig on the hot and cold sides.

Step 1: Calculate the LMTD

Step 2: Calculate NTUHOT and NTUCOLD

Step 3: Read hHot from 0.25 < NTU < 2.0 chart for hydrocarbons

Although is there not a viscosity line for 215 cP, the line representing “100 cP” can be or viscosities up to about 400-500 cP.  The heat exchanger will be pressure drop limited and the heat transfer coefficient will not change appreciably over this viscosity range for plate and frame exchangers.  Reading from the chart, a pressure drop of 15 psig corresponds to hHot @ 50 Btu/h ft2 °F

Step 4: Read hCold from 0.25 < NTU < 2.0 chart for water based liquids

Again, you will note that the exact viscosity line needed for pure water (0.33 cP) in this case is not available.  However, the “1.0 cP” line on the chart will provide a very good estimate of the heat transfer coefficient that pure water will exhibit.  Reading from the chart, a pressure drop of 15 psig corresponds to hCold @ 3000 Btu/h ft2 °F

Step 5:  Calculate the Overall Heat Transfer Coefficient (OHTC)

Assume a stainless steel plate with a thickness of 0.50 mm is being used.  316 stainless steel has a thermal conductivity o 8.67 Btu/h ft °F.

150,000 lb/h of water is being cooled from 200 °°F by 150,000 lb/h of NaCl brine. The brine enters the exchanger at 50 °F and leaves at 171 °F. The average viscosity of the water passing through the unit is 0.46 cP and the average viscosity of the brine in the unit is 1.10 cP. The maximum allowable pressure drop through the plate heat exchanger is 10 psig on the hot (water) side and 20 psig on the cold (brine) side.

As before, the LMTD is calculated to be 38.5 ^°F. NTU_Hot and NTU_Cold are calculated as 2.59 and 3.14 respectively. Reading h_Hot and h_Cold from the chart for 2.0 < NTU < 4.0 (water based), gives about 2000 Btu/h ft^{2 °}F and 2500 Btu/h ft^{2 °}F respectively. Although the material of choice may be Titanium or Palladium stabilized Titanium, we will use the properties for stainless steel for our preliminary sizing. Calculating the OHTC as before yields 918 Btu/h ft^{2 °}F.

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