5. Heat Transfer Equipment Heat Exchanger

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bblee@UniMAP

Heat ExchangerERT 216 HEAT & MASS TRANSFER

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1. Types of Heat Exchangers2. Log Mean Temperature

Difference Correction Factors3. Heat Exchanger Effectiveness4. Fouling Factors and Typical

Overall U values5. Heat Exchanger Design

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Heat exchangers: Heat transfer between 2 fluids.

Common type: The hot & cold fluids do not come into

direct contact with each other but are separated by a tube wall or a flat or curved surface.

Heat transfer from the hot fluid to the wall or tube surface is accomplished by convection, through the tube wall or plate by conduction, & then by convection to the cold fluid.

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Double-pipe (concentric-pipe) heat exchanger: The simplest exchanger, where one fluid

flows inside one pipe and the other fluid flows in the annular space between the two pipes.The fluids can be in co-current or counter-current flow.It is useful mainly for small flow rates.

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Figure 1: Flow in a double-pipe heat exchanger.

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Shell-and-tube exchanger:In these exchangers the flows are continuous.Many tubes in parallel are used, where

one fluid flows inside these tubes. The tubes, arranged in a bundle, are

enclosed in a single shell & the other fluid flows outside the tubes in the shell side.

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The cold fluid enters & flows inside through all the tubes in parallel in 1 pass.

Figure 2: Shell-&-tube heat exchangers: (a) 1 shell pass & 1

tube pass (1-1 exchanger) (a)

(b) 1 shell pass and 2 tube passes (1-2 exchanger).

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1-2 parallel counter-flow exchanger: The liquid on the tube side flows in two

passes as shown & the shell-side liquid flows in one pass.

In the first pass of the tube side, the cold fluid is flowing counter-flow to the hot shell-side fluid; in the second pass of the tube side.

The cold fluid flows in parallel with the hot fluid.

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Cross-flow exchanger:It is commonly used to heat or cool a gas.One of the fluids (liquid), flows inside through the tubes, and the exterior gas flows across the tube bundle by forced or sometimes natural convection. The fluid inside the tubes is considered

to be unmixed, since it is confined and cannot mixed with any other stream.

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Figure 3: Flow patterns of cross-

flow heat exchangers: one fluid mixed (gas) and one

fluid unmixed.

Both fluids unmixed type: It is typically used in air-conditioning

and space-heating applications.

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In this type the gas flows across a finned-tube bundle and is unmixed, since it is confined in separate flow channels between the fins as it passes over the tubes.

Figure 4: Flow patterns of cross-

flow heat exchangers: both fluids unmixed.

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When the hot & cold fluids in a heat exchanger are in true counter-current flow or in co-current (parallel) flow, the log mean temperature difference should be:

The equation holds for a double pipe heat exchanger & a 1-1 exchanger with one shell pass & one tube pass in parallel or counter-flow.

ΔΔΔ

Temperature difference at one

end of the exchanger

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In cases where a multiple pass heat exchanger is involved, it is necessary to obtain a different expression for ∆Tlm.

Depending on the arrangement of the shell & tube passes.∆Tlm which applies either to parallel or to

counterflow but not to a mixture of both types cannot be used without a correction.The usual procedure is to use a correction

factor (FT) which is so defined that when it is multiplied by ∆Tlm, the product is the correct mean temperature drop ∆Tm to use.

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In using the correction factor (FT), It is immaterial the warmer fluid flows through the

tubes or the shell. For a 1-2 exchanger, two dimensionless

ratios are used:

Inlet temperature of hot fluid (K)

Outlet of hot fluid (K)

Inlet of cold fluid (K)

Outlet of cold fluid (K)

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Fig 5: Correction factor (FT) to ∆Tlm for 1-2 & 1-4 exchanger

Fig 6: Correction factor (FT) to ∆Tlm for 2-4 exchanger

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It is not recommended to use a heat exchanger for conditions under FT < 0.75.Another shell and tube arrangement

should be used.

Fig 7: Correction factor (FT) to ∆Tlm for cross flow exchangers

(a) Single pass, shell fluid mixed, other fluid unmixed.

(b) Single pass, both fluids unmixed.

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Then, the equation for an exchanger is:

cohocohi

cihocohi

TTTTln

moomii TAUTAUq ΔΔ

lmTm TFT ΔΔ

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Example 4.9-1:A 1-2 exchanger containing one shell pass

and two tube passes heats 2.52 kg/s of water from 21.1 to 54.4 0C by using hot water under pressure entering at 115.6 and leaving at 48.90C.

The outside surface area of the tubes in the exchanger is A0 = 9.30 m2.

(a) Calculate the mean temperature difference ∆Tm in the exchanger and the overall heat-transfer coefficient U0.

(b) For the same temperatures but using a 2-4 exchanger, what would be the ∆Tm?

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Solution [Example 4-9-1]:(a)

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From Fig. 5:

FT = 0.74

)K(C.).(.TFT o

lmT 331342740ΔΔ

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From Fig. 6:(b) FT = 0.94

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In the design of heat exchangers, ∆Tlm was used in the equation:

This form is convenient when the inlet & outlet temperatures of the two fluids are known or can be determined by a heat balance. The surface area can be determined if U

is known. Heat exchanger effectiveness (ε) is used

which does not involve any of the outlet temperatures.

lmTUAq Δ

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ε is defined as the ratio of the actual rate of heat transfer in a given exchanger to the maximum possible amount of heat transfer if an infinite heat transfer area were available.

The temperature profile for a counter-flow heat exchanger is shown:

Fig 8:Temperature profile for counter-

current heat exchanger

(mcp)H

(mcp)C

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CH>CC and the cold fluid under-goes a greater temperature change than the hot fluid, Cc is designated as Cmin (minimum heat capacity):

If the hot fluid is the minimum fluid, THO = Tci,

CiHimin

HoHimax

CiHimin

CiCOmax

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The denominators of both equations are the same, the numerator gives the actual heat transfer:

For the case of single-pass, counter-flow exchanger,

CiHimin

CiHimin TTCεq

Only inlet temperatures

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We consider the case when the cold fluid is the minimum fluid:

After re-arrangement & solving,

COHiCiHO

COHiCiHOCiCOC

TTTTln

TTTTUATTCq

minmax

11 Graphical form

(See Fig. 9)

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NTU could be defined as the number of transfer units:

For parallel flow,

Same result, CH = C min

11 Graphical form

(See Fig. 10)

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Fig 9: Heat-exchanger effectiveness (ε) for

counter-flow exchanger.

Fig 10: Heat-exchanger effectiveness (ε) for

parallel flow exchanger.

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Example 4.9-2:Water flowing at a rate of 0.667 kg/s

enters a counter-current heat exchanger at 308 K and is heated by an oil stream entering at 383 K at a rate of 2.85 kg/s (cp=1.89 kJ/kg.K).

The overall U = 300 W/m2.K and the area A = 15.0 m2.

Calculate the heat transfer rate and the exit water temperature.

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In actual practice, heat-transfer surfaces do not remain clean.Dirt, soot, scale, & other deposits form

on one or both sides of the tubes of an exchanger and on other heat-transfer surfaces. These deposits offer additional resistance to the flow of heat & reduce the overall heat-transfer coefficient, U. Biological growth (e.g. algae) can occur

with cooling water & in the biological processes.

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Water velocities above 1 m/s are generally used to help reduce fouling.

Large temperature differences may cause excessive deposition of solids on surfaces and should be avoided if possible.

The effect of such deposits & fouling is usually taken care of in design by adding a term for the resistance of the fouling on the inside & outside of the tube:

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hdi: The fouling coefficient for the inside (W/m2.K).

hdo: The fouling coefficient for the outside of the tube (W/m2.K).

Table 1: Typical Fouling Coefficients

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Table 2: Typical values of overall heat-transfer coefficients in shell-and-tube exchangers

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A substantial number of parameters is involved in the design of a shell-and-tube heat exchanger for specified thermal and hydraulic conditions and desired economics, including:

tube diameter size of shell length

number of passes

number of shell baffles

square or triangular

baffle type thickness

baffle windows baffle spacing

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A logic diagram of a heat exchanger design procedure appears in Fig. 11.

Fig 11: A procedure for the design of a heat exchanger, comprising a tentative selection of design parameters

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The key elements of heat exchanger design are:i. Selection of a tentative set of design

parameters. ii. Rating of the tentative design, which

means evaluating the performance with the best correlations and calculation methods that are feasible.

iii. Modification of some design parameters, then rerating the design to meet thermal and hydraulic specifications and economic requirements.

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Stainless Steel Tubes:The simplest method to increase heat

transfer is to increase the number of tube-side passes, if the controlling resistance to heat transfer is on the tube side and current tube-side velocities are low (< 3 ft/s).Other than high velocities, the next best

method to control fouling on the both shell and tube sides is to prevent the formation of rough surfaces due to corrosion.

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A smooth, mirror-finished surface will retard the accumulation of fouling deposits.

Two notes of caution regarding re-tubing with stainless:i.Do not put stainless tubes in direct

physical contact with carbon steel tube support baffles/carbon steel tube sheets. The results will be galvanic corrosion of

the carbon steel components. nine chrome tubes are consistent with

carbon steel components.

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ii. 304, 316 and 317 stainless should not be used in crude preheat.

At least, not upstream of the desalter. The problem is chloride stress corrosion

cracking.

Sintered Metal Tubes:Rough surfaces are bad for sensible heat

transfer due to fouling.But in clean services, rough surfaces are

critically important to boil water and hydrocarbons.

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Rough surfaces provide nucleation sites for bubbles to form.

For one butane reboiler, an old reboilerbundle with pitted carbon steel tubes had double the heat transfer capacity of a new bundle.

There are two ways to prevent loss of heat transfer on reboilers when a newly retubedbundle is commissioned. A sintered metal coating can be applied to

the tubes, or the tubes can be lightly sand blasted to

roughen their surface.

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Tube Inserts:These are springs that are inserted into

the tubes. Some types are fixed and some spin with

the flow. The objective is to create turbulence

that reduces the tube-side film resistance, and the rate of tube-side fouling.

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Twisted Tubes:It looks like a 1-inch hollow drill bit. The idea is that the twisted surface

generates more turbulence on both the tube side and the shell side than a straight tube.

The twisted tube does not require any tube support baffles.

The tubes touch and thus are self-supporting.

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However, without any tube support baffles, an external sleeve or shroud is needed to keep the tubes in place.

Care must be taken in handling the shroud so as not to alter the alignment of the twisted tubes.

Fig 12:A twisted tube. Bundle does not have tube support baffles.

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Helical Tube Support Baffles:The flow through the tube side of this

type of exchanger is conventional. However, the shell-side flow is unique. It is neither perpendicular to the

tubes nor parallel. Rather, the shell-side flow follows a

screw-type pathway across the tubes. It is the angled slope of the baffles that

induce this sort of helical or screw-type flow to the shell-side liquid.

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Fig 13:Tube bundle with helical tube support baffles. Liquid flows in a screw type path.

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The advantage of this sort of flow is that dead zones are eliminated in the exchanger areas where ordinarily the flow changes directions along the edge of the tube support baffles.

For this strategy to work, the controlling resistance to heat transfer must be on the shell side.

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Practical ideas to enhance heat transfer efficiency:

Several more conventional methods to enhance heat transfer are:

i. Minimize clearance between tube support baffles and shell ID, as per TEMA specifications.

ii. Use ½-inch rather than the standard ¼-inch space between tubes. o This will reduce dirt bridging between

tubes on the shell side. o This bridging problem creates dead

zones with no flow and no heat transfer.

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iii.Specify the highest possible allowable pressure drops on the exchanger data sheets. o This permits vendors to design for a

high velocity, which suppresses fouling rates.

iv. Include block valves and bypasses to stop flow for brief periods. o The result is thermal spalling and/or

melting of deposits from tube surfaces.

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v. Use floating head, not U tube exchangers. o U tube exchangers cannot be visually

inspected after cleaning the tube side of the bundle.

vi. Do not rerun cracked recovered slops through heat exchangers after the slops have been exposed to air.o Polymerization will result at about

300°F to 350°F. The polymers form gums which promote fouling.

o Virgin materials do not polymerize.

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vii. Use vertically cut baffles in boiling or condensation services for the shell-side flow.

viii.Charge from tanks with floating suctions. o Dirt will settle out of the bottom of

the tanko Wait until just before a unit

turnaround to start the internal tank mixers.

o Exchangers will then foul rapidly.

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ix. Watch for hydrocarbon leaks in cooling water system. o Hydrocarbons promote biological fouling inside the tubes.

x. Elevate condensers above reflux drums for drainage to avoid condensate backup.

xi. Place the fluid with the lowest Reynolds number (i.e., the high viscosity fluid) on the shell side. o the most important point

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o The resulting vortex shedding as the liquid flows perpendicularly across the tubes will avoid laminar flow and the resulting high heat transfer film resistance.

o As long as the tube pitch is rotated square, the shell will still be able to be cleaned even though the shell-side flow is dirty.

5. Heat Transfer Equipment Heat Exchanger

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