Shell and tube heat exchanger

SHELL AND TUBE HEATEXCHANGER THERMAL DESIGN

The general equation for heat transfer across a surface is:

Q = U.A.ΔTm

Where              Q = heat transferred per unit time, W,

                        U = the overall heat transfer coefficient, W/m2 ºC,

                        A = heat-transfer area, m2,

                        ΔTm =  the mean temperature difference, the temperature driving force, ºC.

The overall coefficient:-

Where              U_o = the overall coefficient based on the outside area of the tube, W/m2 ºC,

                        h_o = outside fluid film coefficient, W/m2 ºC,

h_i = inside fluid film coefficient, W/m2 ºC,

h_od = outside dirt coefficient (fouling factor), W/m2 ºC,

h_id = inside dirt coefficient, W/m2 ºC,

k_w = thermal conductivity of the tube wall material, W/m2 ºC,

d_i  = tube inside diameter, m,

d_o = tube outside diameter, m.

The magnitude of the individual coefficients will depend on the nature of the heat transfer

Process , on the physical properties of the fluids, on the fluid flow-rates, and on the physical arrangement of the heat-transfer surface. As the physical layout of the exchanger cannot be determined until the area is known the design of an exchanger is of necessity a trial and error

Procedure .

Design Procedure of heat exchanger:-

Step 1.  Obtain the required thermophysical properties of hot and cold fluids at the caloric temperature or arithmetic mean temperature. Calculate these properties at the caloric temperature if the variation of viscosity with temperature is large.

Step 2.   Find out the heat duty (Q) of the exchanger.

Heat duty for liquid phase heat exchanger:-

Q = m . Cp . ( To-Ti )

Where :-

m = Mass flow rate in Kg/s.

C_p = Specific heat of liquid in J/Kg

T_i = Inlet temperature in K

T_o = Outlet temperature in K

Heat duty for vapour phase heat exchanger:-

Q = mL + m . Cp . ( To-Ti )

L= Latent heat of vaporization J/kg

Step 3. Select a value of overall heat transfer coefficient (U_oas). The value of U_oas with respect to the process hot and cold fluids can be obtain from HERE.

Step 4. Decide tentative number of shell and tube passes (n_p). Determine the LMTD and the correction factor. F_T normally should be greater than 0.75 for the steady operation of the exchangers. Otherwise it is required to increase the number of passes to obtain higher F_T values.

LMTD for counter flow:-

 

The equation is the same for co-current flow, but the terminal temperature differences

will be (T1- t1) and (T2 - t2).

To calculate F_T

_{CLICK HEAR}

Step 5. Calculate heat transfer area (A) required:-

A = Q / U_{oas .}LMTD  F_T

Step #6. Select tube material, decide the tube diameter (ID= d_i , OD = d_o ), its wall thickness (in terms of BWG or SWG) and tube length ( L ). Calculate the number of tubes (n_t) required to provide the heat transfer area (A):-

                                                n_t = A / πd_oL

Calculate tube side fluid velocity, u =4 m ( n _p / n_t ) / prd_i ²

If u <1 class="Apple-converted-space" fix="" m="" s="" span=""> 

n _p so that, Re = 4 m ( n _p/ n_t ) / π d_iµ ³10⁴

Where, m, r and m are mass flow rate, density and viscosity of tube side fluid. However, this is subject to allowable pressure drop in the tube side of the heat exchanger.

Step #7. Decide type of shell and tube exchanger (fixed tubesheet, U-tube etc.). Select the tube pitch (PT), determine inside shell diameter (D_s) that can accommodate the calculated number of tubes (n_t ).

To Select tube pitch CLICK HERE.

Step #8. Assign fluid to shell side or tube side (a general guideline for placing the fluids is given below). Select the type of baffle (segmental, doughnut etc.), its size (i.e. percentage cut, 25% baffles are widely used), spacing ( B ) and number. The baffle spacing is usually chosen to be within 0.2 D_s to D_s .

Tube-side fluid		Shell-side fluid
Corrosive fluid		Condensing vapor (unless corrosive)
Cooling water		Fluid with large temperature difference (>40°C)

      Fouling fluid

           Less viscous fluid

           High-pressure steam

           Hotter fluid

To Select Baffle size CLICK HERE

Step #9. CALCULATE TUBE-SIDE HEAT-TRANSFER COEFFICIENT(hi) AND PRESSURE DROP. CLICK HERE

Step #10. CALCULATE SHELL-SIDE HEAT-TRANSFER (ho) AND PRESSURE DROP  

                 CLICK HERE

Step #11. CALCULATE OVERALL HEAT TRANSFER COEFFICIENT ( Uo ) BY 

Take hi and ho from stem 9 and 10.

h_{od ,} h_{id  &} k_w

_{Select From Table}

       

     Typical values of fouling coefficients and resistances 

	Fluid	Coefficient (W.m-2.°C-1)	Resistance (m2.°C.W-1)
	River water	3000-12,000	0.0003-0.0001
	Sea water	1000-3000	0.001-0.0003
	Cooling water (towers)	3000-6000	0.0003-0.00017
	Towns water (soft)	3000-5000	0.0003-0.0002
	Towns water (hard)	1000-2000	0.001-0.0005
	Steam condensate	1500-5000	0.00067-0.0002
	Steam (oil free)	4000- 10,000	0.0025-0.0001
	Steam (oil traces)	2000-5000	0.0005-0.0002
	Refrigerated brine	3000-5000	0.0003-0.0002
	Air and industrial gases	5000-10,000	0.0002-0.000-1
	Flue gases	2000-5000	0.0005-0.0002
	Organic vapors	5000	0.0002
	Organic liquids	5000	0.0002
	Light hydrocarbons	5000	0.0002
	Heavy hydrocarbons	2000	0.0005
	Boiling organics	2500	0.0004
	Condensing organics	5000	0.0002
	Heat transfer fluids	5000	0.0002
	Aqueous salt solutions	3000-5000	0.0003-0.0002

Step #12. if 0 <  (Uo-Uoas) / Uoas  > 30 % Design is OKAY otherwise repeat design with Uo . and calculate area.