Shell and Tube Heat Exchanger

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INSTRUCTION MANUAL.

SHELL AND TUBE HEAT EXCHAGER SET UP

 DESCRIPTION  SPECIFICATIONS

 INSTALLATION REQUIREMENTS  INSTALLATION AND CONNECTIONS  PRECAUTIONS  TROUBLE SHOOTING EXPREMENTATION  THEORY  OPERATING PROCEDURE’  SYSTEM CONSTANTS  OBSERVATION TABLE  CALCULATIONS

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SHELL AND TUBE HEAT EXCHANGER. AIM:

To determine the overall and individual heat transfer coff. In 1.2 shell & tube heat exchanger.

INTRODUCTION:

Shell & tube heat exchangers are mostly used in chemical industries as a condenser, cooler and heater. When the heat transfer are becomes more than 10 m², one has to go for shell & tube heat exchanger, which occupies less space, offers reasonable pressure drop, compared to other types of heat exchangers. Because of its wide utility. Mechanical design has been standardized. Standards of TEMA are available covering in details of materials, methods of construction, technique of design, and determinations for exchangers.

THEORY:

The film heat transfer coff. Is a function of cross-sectional area of the fluid path. Thus, decreasing the cross sectional area could increase the fluid velocity. This is achieved in multi pass heat exchangers at the cost of pressure drop & temperature gradient affecting due to concurrent flow. The temperature profile for 1-2-pass heat exchangers is shown in the fig. 1. The factor Ft is the temperature difference correction factor, when it is multiplied by the counter flow LMTD; the product is correct mean temperature difference. Fig.2 shows the factor Ft as a function of two dimentionsless numbers h & z, which are defined as

Tco - Tci Z = Thi - Tho Tho - Thi Z = Tci - Tco

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The factor h is the heating effectiveness of the ratio of the actual temperature rise of the cold fluid to the maximum possible temperature rise if the warm end approach were zeros. (Based on counter current flow.) The factor Z is the ratio of fall in temperature of the hot fluid to the rise in temperature of the cold fluid. Interpolation is permitted I using fig.2.

By keeping the flow rate and physical properties constant in the shell side fluid and varying the flow rate of the tube side fluid, and applying Wilson’s plot it is possible to get the outside (shell side) film heat transfer coff. As subsequently the inside (tube side) film heat transfer coff.

EXPERIMENTAL SET – UP

A standard 1-2 pass shell and tube heat exchanger consist of (A) Shell : 1) I.D. = 154 mm 2) Thickness = 6 mm 3) Material = MS 4) Baffle = 25% cut. 5) Baffle spacing = 57mm. (B) Tube : 1) OD =12.5 mm 2) Thickness = 1.5 mm 3) Pitch – 20 mm triangular 4) No. Of tubes = nos. 5) Length – 1000 mm

The hot water tank (200 ltrs cap.) is provided with 6 kWh. Heaters and hot water is pumped by centrifugal pump. The flow rates of the fluids are known by pre calibrated rotameters. The cold water tank is of 200 ltrs, Capacity with centrifugal pump. Thermometers are provided at

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inlet & outlets of the heat exchanger to know temperature of the shell & tube sides by pass valves are provided to vary flow rates.

PROCEDURE:

1. Admit water into hot & cold water tanks and keeps heater on.

2. When the temperature of hot water reaches to 60 to 70 C, admit it into tube side of the exchanger at fixed known flow rate.

3. Admit cold water into shell side of the heat exchanger. At steady state. Note down temperature and flow rates of the shell and tube side fluids.

4. Keep the flow rate and temp. Of hot fluid constant and vary flow rate of cold water at uniform intervals. Repeat the above procedure and note down readings.

Sr. Shell side fluid tube side fluid

1. 2. 3. 4. 5. Flow rate outlet temp Inlet temp.

Flow rate outlet temp Inlet temp. Kg/hr.Mc  C t2  C t1 Kg/hr.Mc  C T2  C T1 ENERGY BALANCE: Q = mh. Cph (T1 – T2) = mc .Cpc (t2 – t1)

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 Tlm = (T1 – t2) – ( T2 – t1) ln (T1 – t2) (T2 – t1) R = (T1 – T2) (T2 - t1) S = ( T2 – t1) ( t2 – t1).

Find out factor Ft from graph (Process heat transfer by KERN page no. 828)

t = Ft X Tlm.

COLD FLUID ( SHELL SIDE) As = ID X C’B /Pt.

Gs = W/As.

At Ta ( average temp. of cold fluid t1 +t2/2) find out  . To find out equivalent dia. De refer fig. 28 page 828. Res = De. Gs/.

To find out factor Jh i.e. factor for heat transfer (dimentionless) refer fig. 28 page 828.

At Ta = t1+ t2/2 find out specific heat of fluid, k-cal/kg-c. Hence thermal conductivity k= k-cal/kgc.

1/3 (c/k).

1/3 ho = jh X k/De X (c/k). X 1. HOT FLUID (TUBE SIDE)

Flow Area per tube A’t = П/4 di² …m².

Flow area of fluid = no of tubes X flow area per tube/no. of passes. … m².

Now mass velocity Gt = mass flow/ area = w/at. Kg/hr.m². Velocity = Gt/3600ρ. Where ρ = density kg/m³.

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D = inside dia. Of tube = m. Re = DGt/.

Calculate Hi from fig. 25 page 828 . Hio = Hi X ID/OD. Now CLEAN OVERALL HEAT TRANSFER COFFICIENT Uc = HioX Ho

HioxHo.

Design overall heat transfer coff.Ud =

= Q/At. here A = external surface /m. dirt factor = Rd = Uc – Ud/Uc X Ud.

PRESSURE DROP. SHELL SIDE.

Find out factor f, for shell side Re. f = m²/cm.

No. of crosses = N+1 = 12L/B where L= tube length m , B = baffle spacing m.

Ds= inside dia. Of shell. M . Ps = 8jf(Ds/de)(L/lb)ρ²/2. Tube side pressure drop.

= Pt = 8jf (L’/di). t²ρ/2. N/m². DISCUSSION:

Energy balance gives

Q = mh Cph ( t hi – t ho ) = mc Cpc ( t co – t ci )

Which is related with overall heat transfer coefficient by, Q = Uo Ao Δtlm X FT.

Flow area per tube = a’t = /4 dt², m².

at = flow area of fluid = no. of tubes X flow area/tube No. of passes.

= Nt a’t /n m². Now mass velocity of the tube side fluid, = Gt = mc/at, and get Rei = dt Gt/µ.

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Now Gt = Vi/. Vi = Gi/ m/hr.

 plot the graph of 1/Uo Vs 1/Vi

Figure

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References

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