Heat transfer Equipment:Plate Heat Exchanger

• A gasketed plate heat exchanger consists of a stack of closely spaced thin plates clamped together in a frame.

• A thin gasket seals the plates round their edges.

• The plates are normally between 0.5 and 3 mm thick and the gap between them 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 maximum flow-rate of fluid is limited to around 2500 m3/h.

• The basic layout and flow arrangement for a gasketed plate heat exchanger is shown in next page.

• Plates are available in a wide range of metals and alloys; including stainless steel,aluminium and titanium.

• Selection• The advantages and disadvantages of plate heat exchangers,

compared with conventional shell and tube exchangers are listed below:

• Advantages 1. Plates are attractive when material costs are high. 2. Plate heat exchangers are easier to maintain. 3. Low approach temps can be used, as low as 1 C, compared with 5 to 10 C for shell and tube exchangers. 4. Plate heat exchangers are more flexible, it is easy to add extra plates. 5. Plate heat exchangers are more suitable for highly viscous materials. 6. Fouling tends to be significantly less in plate heat exchangers; see Table 1.

Table 1.Fluid Coefficient (W/m2 °C) Factor (m2 °C/W)

Process water 30,000 0.00003

Towns water (soft) 15,000 0.00007

Towns water (hard) 6000 0.00017

Cooling water (treated) 8000 0.00012

Sea water 6000 0.00017

Lubricating oil 6000 0.00017

Light organics 10,000 0.0001

Process fluids 5000 20,000 0.0002 0.00005

Disadvantages

• 1. A plate is not a good shape to resist pressure and plate heat exchangers are not suitable for pressures greater than about 30 bar.

• Plate heat exchangers are used extensively in the food and beverage industries, as they can be readily taken apart for cleaning and inspection. Their use in the chemical industry will depend on the relative cost for the particular application compared with a conventional shell and tube exchanger

• 2. The selection of a suitable gasket is critical; see table 2.

• 3. The maximum operating temperature is limited to about 250 C, due to the performance of the available gasket materials.

Plate heat exchanger design• It is not possible to give exact design methods for plate heat

exchangers. They are proprietary designs, and will normally be specified in consultation with the manufacturers.

• Information on the performance of the various patterns of plate used is not generally available.

• The approximate method given below can be used to size an exchanger for comparison with a shell and tube exchanger, and to check performance of an existing exchanger for new duties.

ProcedureThe 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,LMTD. 4. Determine the log mean temperature correction factor, Ft 5. Calculate the corrected mean temperature difference6. Estimate the overall heat transfer coefficient; see Table3.

• 7. Calculate the surface area required;

• 8. Determine the number of plates required D total surface area/area of one plate.• 9. Decide the flow arrangement and number of passes.

Flow arrangementsThe stream flows can be arranged in series or parallel, or a combination of series andparallel, see Figure 1. Each stream can be sub-divided into a number of passes;analogous to the passes used in shell and tube exchangers.

• Flow arrangements

• Estimation of the temperature correction factor

• For plate heat exchangers it is convenient to express the log mean temperature difference correction factor, Ft, as a function of the number of transfer units, NTU, and the flow arrangement (number of passes); see Figure 2. The correction will normally be higher for a plate heat exchanger than for a shell and tube exchanger operating with the same temperatures. For rough sizing purposes, the factor can be taken as 0.95 for series flow.

• 10. Calculate the film heat transfer coefficients for each stream; see method given below.

Heat transfer coefficient• The equation for forced-convective heat transfer in conduits

can be used for plate heat exchangers; equation3.

Eq3.• The values for the constant C and the indices a,b,c will depend

on the particular type of plate being used. Typical values for turbulent flow are given in the equation below, which can be used to make a preliminary estimate of the area required.

• The corrugations on the plates will increase the projected plate area, and reduce the• effective gap between the plates. For rough sizing, where the actual plate design is not• known, this increase can be neglected. 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.

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

• 12. Compare the calculated with the assumed overall coefficient. If satisfactory, say 0% to C 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

• Pressure drop

• The plate pressure drop can be estimated using a form of the equation for flow in a conduit; see equation below

• The value of the friction factor, jf, will depend on the design of plate used. For

• preliminary calculations the following relationship can be used for turbulent flow:

• The transition from laminar to turbulent flow will normally occur at a Reynolds number of 100 to 400, depending on the plate design. With some designs, turbulence can be achieved at very low Reynolds numbers, which makes plate heat exchangers very suitable for use with viscous fluids.

• The pressure drop due the contraction and expansion losses through the ports in the plates must be added to the friction loss.

Example

• Investigate the use of a gasketed plate heat exchanger. Cooling methanol using brackish water as the coolant. Titanium plates are to be specified,to resist corrosion by the saline water.

• Cool 100,000 kg/h of methanol from 95°C to 40°C, duty 4340 kW. Cooling water inlet temperature 25°C and outlet temperature 40°C. Flow-rates: methanol 27.8 kg/s, water 68.9 kg/s.

• Logarithmic mean temperature difference 31°C

Physical properties: Methanol Water

Density, kg/m3 750 995

Viscosity, mN m-2s 3.4 0.8

Prandtl number 5.1 5.7

• Solution• NTU, based on the maximum temperature

difference(95-40)/31=1.8

• Try a 1 : 1 pass arrangement.From Figure2, Ft = 0.96• FromTable 3 take the overall coefficient, light

organic - water, to be 2000 Wm-2 0C-1

• Then, area required(4340*103)/(2000*0,96*31)=72,92 m2

• Select an effective plate area of 0.75 m2, effective length 1.5 m and width 0.5 m; these are typical plate dimensions. The actual plate size will be larger to accommodate the gasket area and ports.

Number of plates = total heat transfer area / effective area of one plate 72.92/0,75=97

• No need to adjust this, 97 will give an even number of channels per pass, allowing for an end plate.

• Number of channels per pass =(97 - 1)/2 = 48• Take plate spacing as 3 mm, a typical value, then:• channel cross-sectional area = 3*10-3*0,5=0,0015 m2

• and hydraulic mean diameter =2*3*10-3=6*10-3 m

• Methanol• Channel velocity=(27,8/750)*(1/0015)*(1/48)=0,51m/s• Re =(750*0,51*6*10-3)/(0,34*10-3)=6750• Nu=0,26*(6750)0,65 *5,10,4 =153,8• hp =153,8*(0,19/6*10-3)=4870Wm2 oC-1

• Brackish water• Channel velocity=(68,9/995)*(1/0,0015)*(1/48)=0,96m/s• Re=(995*0,96*6* 10-3)/(0,8* 10-3)=6876• Nu=0,26*(6876)0,65 * 5,70,4 =162,8• hp= 162,8*(0,59/(6* 10-3))=16,009 Wm2 oC-1

• Overall coefficient• From Table 1, take the fouling factors (coefficients) as:

brackish water (seawater) 6000Wm2 oC-1 and methanol (light organic) 10,000 Wm2 oC-1.

• Take the plate thickness as 0.75 mm. Thermal conductivity of titanium 21 Wm-1 oC-1.

• 1/U=(1/4870)+(1/10000)+((0,75*10-3)/21)+(1/16,009)+(1/6000)U=1754 Wm2 oC-1, too low

• Increase the number of channels per pass to 60; giving ; (2 * 60) +1 = 121 plates.

• Then, methanol channel velocity = 0.51 * (48/60) =0.41 m/s, and Re =5400.

• Cooling water channel velocity = 0.96 * (48/60)= 0.77 m/s, and Re =5501.

• Giving, hp =4215 Wm2 oC-1for methanol, and 13,846 Wm2 oC-1 for water.

• Which gives an overall coefficient of 1634 Wm2 oC-1.• Overall coefficient required 2000 * (48/60) = 1600

Wm2 oC-1, so 60 plates per pass should be satisfactory.

• Pressure dropsMethanolJf=0,60*(5400)-0,3 =0,046Path length =plate length = number of passes = 1.5 * 1 = 1.5 m.ΔPp =8*0,046*(1,5/6*10-3)*750*((0,412)/2)=5799N/m2

• Port pressure loss, take port diameter as 100 mm, area=0.00785 m2.

• Velocity through port=(27,8/750)/0,00785=4,72m/s,• ΔPpt= 1,3*((750*4,722)/2)=10860 N/m2

• Total pressure drop = 5799 + 10,860 = 16,659 N/m2 , 0.16 bar.

• Water• Jf=0,60*(5501)-0,3=0,045• Path length = plate length * number of passes = 1.5 *1 =1.5 m.• ΔPp =8*0,045*(1,5/6*10-3)*995*(0,772/2)=26547N/m2

• Velocity through port =(68.9/995)/0.0078 = 8.88 m/s• ΔPpt =1,3*(995*8,88)/2=50999N/m2

• Total pressure drop = 26,547 +50,999= 77,546N/m2, 0.78 bar

• Could increase the port diameter to reduce the pressure drop.• The trial design should be satisfactory, so a plate heat exchanger

could be considered• for this duty.