Efficiency of Heat Exchangers

A PROJECT WORK ON

EFFECTIVENESS OF PRE-HEAT

EXCHANGERS)

SUBMITTED BY

MOHAMMED ABDUL GHAFOOR

B-TECH 4TH YEAR (PETROLEUM ENGINEERING)

EFFECTIVENESS OF PRE-HEAT

EXCHANGERS

As part of Internship / Project Work to have an insight in to the Oil Refinery carried out study on – “OFFECTIVENESS OF PRE-HEAT EXCHANGERS”.

Session: June-July (2015). Internship / Project work carried . Under the Guidance , Deputy Production Manager (DPNM) DHDS, IOCL.

ABSTRACT

The Preheat Exchangers are a series of heat exchanger trains in which the heat gets exchanged between the feed and the hot reactants coming from the reactor 01 of the high pressure section. The efficiency (η) of 25-E-02 and 25-E-04 can be increase up to the designed conditions by using some enhancement technique on the tube side such as Finned surface. In the above mentioned exchangers Over all heat transfer coefficient is usually dominated by smaller hear transfer coefficient. Note that, the rate of heat transfer decreases as a result of fouling as expected.

The decrease is not dramatic because of relatively low convection heat transfer coefficient. The efficiency can be enhanced by proper maintenance of heat exchanger like proper operation, Corrosion inhibitor, furnace firing control etc. The material used in the construction of heat exchanger may be an important consideration in the selection of heat exchanger. We may have to select expensive corrosion resistant materials such as stainless

steel or even titanium if we are not willing to replace low cost heat exchangers frequently.

INTRODUCTION :

In many process industries, it is frequent necessity to heat, cool, vaporize or condense various fluid streams. Different types of heat exchangers are used for such purposes. A heat exchanger is a device in which two fluid streams, one hot and another cold are brought into thermal contact in order to effect transfer of heat from hot fluid to the cold. It provides a relatively large area of heat transfer for a give volume of equipment.

A shell and the exchanger is most widely used heat exchanging equipment. We shall first describe the constructional features of this type of heat exchanging equipment. This will be discussed on a few types of heat exchangers which are frequently used in the chemical processing plant as well as in the refrigeration, cryogenic, waste heat recovery, metallurgical and metallic manufacturing applications. heat exchangers can be classified on the basis of contacting techniques, constructions, flow arrangement or surface compactness.

Most of the heat exchangers are indirect type in which the hot and cold fluids are in thermal contact but physically separated by a barrier such as tube wall, plate etc. in a direct contact unit the fluids are brought in contact with upflowing air is a common example of direct heat exchanger.

MODES OF HEAT TRANSFER:

Conduction Convection Radiation

CONDUCTION:Flow of heat in a substance due to exchange of energy between the molecules having more

energy and molecules having less energy.

CONVECTION:The transfer of energy from one region to another due to macroscopic motion in a fluid added to the heat energy transferred by conduction is called Convection.

It is of two types,

Forced Convection- fluid motion caused by external agency.

Natural Convection- fluid motion occurs due to density variation caused by temperature difference.

RADIATION:

All physical matters emits thermal radiations in the form of electromagnetic waves because of vibration and rotational movement of the molecules and atoms which makes up the matter. The rate of emission increases with increase in temperature level. No material medium required for energy transfer to occur.

TYPES OF HEAT EXCHANGERS:

DIRECT TRANSFER TYPE STORAGE TYPE DIRECT CONTACT TYPE

Direct transfer type in which the cold and hot fluid flows simultaneously through the device and heat is transferred through a wall separating the fluids.

MAJOR COMPONENTS:

Tubes Shells Baffles Nozzles Tube Sheets Expansion joints

Tubes:

The tube provides the heat transfer area in she and tube heat exchanger. One of the fluid flows through the tube and the other flows through the shell along the outside of the tubes. The fluids are brought in thermal contact through the tube wall. Both seamless and welded tubes are used and a wide variety of materials including low carbon steel, stainless steel, nickel , copper, brass, aluminum etc. are used. The choice of the tube is depend on the nature of the applications fined tubes are sometimes used and the fin efficiency up to 90% may be attained.

Shells:

The shell is the enclosure and passage of shell side fluid. It has a circular cross section and is made by rolling a metal of suitable dimension into a cylinder and welding along the length.

However, a section of a pipe of suitable thickness can be used as a shell if the diameter is not large up to 60cm. the selection of the material is depends upon the corrosiveness of the fluid and working temperature and pressure.

Tube Sheets:

The tube sheets are circular thick metal plates which hold the tube at ends. In the type of the exchanger called fixed tube sheet are welded to the shell at the ends. The tubes are inserted into holes of two tubes sheets. Perfect alignment of the holes is required; this is achieved by drilling tube holes through the sheets fastened together for the purpose, particularly for the small thickness tube sheets. The diameter of the tube hole is made up of slightly larger than tube diameter.

Nozzles:

Small sections of pipes welded to the shells or to the channel which acts as the inlet or outlet to the fluids are called Nozzles. The end of the nozzles is flanged to connect it to the pipe carrying a fluid. The shell side inlet nozzle is often provided with an impingement plate it prevents the impact of high velocity inlet fluid streams on the tube bundles. Such impact can cause the erosion and cavitations of the tubes just in front of the nozzle and can also cause vibration of the tubes.

PRE-HEAT EXCHANGERS IN DHDS UNIT

The Pre-Heat exchangers in the DHDS unit are used to exchange the heat from the reactor effluents coming from the Reactor-01 and Reactor-02. The unit is divided into two sections (i.e.) High Pressure Section and Low Pressure Section. The high pressure section consists of Pre-Heat Exchangers, Reactors, Furnace, Air Coolers and High Pressure Separator.

After the feed pumps, 25-p-01A/B, The reactor gas-oil is divided into two lines and then mixed with recycle gas hydrogen, coming from recycle compressor. Then, the gas oil feed is preheated by reactor effluent in a series of heat exchangers before entering the reactor charge heater or furnace (25-F-01) where heat is recovered from reactor effluents by feed (HC+H2Gas+H2S).

Mixed feed first enters the cold combined feed exchanger (25-E-04) and then hot combined feed exchanger (25-E-02). In order to control the outlet temperature a cold feed line is bypassed in between the two exchangers.

The two bed reactor support is separated by a quench section (gas distributor) mixing internals and a vapor/liquid re-distributor tray and due to exothermic reactions taking place in reactor the outlet temperature is more than the inlet temperature. The heat is recovered by a series of heat exchangers

Hot combined feed exchangers 25-E-02 Stripper feed effluent exchangers 25-E-01, 25-E-03 and 25-E-05, heating stripper column

feed; a cold stripper by pass is foreseen to control the stripper column inlet temperature. Cold combined feed exchangers 25-E-04, heating charge heater feed.

ARITHMATIC OPERATION OF PRE-HEAT EXCHANGERS

Designed Parameters

STRIPPER BOTTOM FEED EXCHANGER: [ 25-E-01]

The designed operating conditions of stripper bottom feed exchanger

Effectiveness (η)= (tc2-tc1) / (th2-tc1)

= (74-40) / (118-40)

=0.435 (43.5) %

Capacity ratio = (th1-th2) / (tc2-tc1)

= (118-65) / (74-40)

=1.558

Where th1= hot inlet temp at tube side

Parameter SHELL SIDE TUBE SIDE

Flow rates (kg/h)

165000 160347

Temp. oC 40o C, 74o C 118o C, 65oC

Density(kg/m3) 842.5 800

Viscosity Cp 3.279 1.4195

Sp.Heat Kcal/kg oC

0.458 0.509

Thermal Cond.(k)

0.106 0.1005

th2= hot outlet temp at tube side

tc1= cold inlet temp at shell side

tc2= cold outlet temp at shell side

Heat Duty (Qh) = m.Cp.∆T (for hot side)

= 160437x0.509x(118-65)

= 3892263.078kj/h

= 929873.16kcal/h

Heat Duty (Qc) = m.Cp.∆T

= 165000x0.458x (74-40)

= 2669380kj/h

= 613832.48kcal/h

Over all heat transfer coefficient with fouling factor at 0.0004

As we have calculated the inside and outside film heat transfer coefficients as hi and ho.

H i = 1961.358 w/m2 C

Ho= 530216 w/m2 C

The over All heat transfer coefficient with fouling factor is written as

1/U= (1/ho) + Rd + (Ao/Am) x (ro-ri) / Kw + (Ao//Ai) x Rd+(Ao//Ai)x(1/hi)

1/U= 0.01725795 w/m2C,

Now,1/Ud0= (1/U)+Rd

= 57.994+0.0004

= 57.994w/m2 K

Operating Conditions:

SHELL SIDE TUBE SIDE

Flow rate kg/h 165000 160437Operating temp oC 43o C , 79.8oC 119oC , 84.6oCDensity kg/m3 854 790Viscosity Cp 2.187 1.114Sp.heat kcal/kg oC 0.474 0.495Thermal cond. K 0.104 0.102


= (79.8-43) / (119-84.6)

= 048 (48%)


= (119-84.6) / (79.8-43)

= 0.9347


= 160347x1.114x(119-84.6)

= 6144753kj/hr

= 1467999.6kcal/hr

Heat Duty (Qc)= m.Cp.∆T

= 165000x2.187x(79.8-43)

= 13729464kj/hr

3280009.556kcal/hr

Over all heat transfer coefficient with fouling factor at 0.0004 :

Now, Uo= 52.1704w/m2 K

Hi= 10992.15 w/m2 oC

Ho= 51.59 w/m2 oC

Ui=73.12w/m2 K

Designed parameters:

HOT COMBINED FEED ECHANGER [25-E-02]


Flow rate kg/h 78179 90394Temperature oC 237, 340 368, 301Density kg/ m3 666.5 623Viscosity Cp 0.295 0.2365Sp.heat kcal/kg oC 0.644 0715Thermal cond.(k) 0.0775 0.072

Effectiveness(η)= (tc2-tc1) / (th2-tc1)

= (340-237) / (368-237)

= 0.786 (78.6%)


= (368-301) / (340-237)

= 0.6504


= 90394x0.715x(368-301)

= 4330324.57kj/hr

= 1034527.347kcal/hr

Heat Duty (Qc)= m.Cp.∆T ( for cold side)

= 78179x0.644x(340-237)

= 5346818.16kj/hr

= 1277370.672kcal/hr

Over all heat transfer coefficient with fouling factor is given as

Now,1/Ud0= (1/U)+Rd

= 52.1704w/m2 K

Hi= 10992.15 w/m2 oC

Ho= 51.59 w/m2 oC

Operating Parameters:


Flow rate kg/hr 78179 90394Temperature oC 189 oC, 345 oC 363 oC , 259 oCDensity kg/m3 709 596Viscosity Cp 0.37 0.206Sp.heat kcal/kg oC 0.622 0.742Thermal cond.(k) 0.084 0.068

Effectiveness (η) = (tc2-tc1) / (th2-tc1)

= (314.5-189) / (363-189)

=71.8%


= 0.8286


= 90394x0.742x(363-259)

=6975524kj/hr

= 1666473kcal/hr

Heat Duty (Qc) = m.Cp.∆T (for cold side)

= 3615778Kj/hr.

=863820.30kcal/hr

Over all heat transfer coefficient with fouling factor as 0.0004,

Hi= 10103.15w/m2 oC

Ho= 40.98w/m2 oC

Uo= 52.170w/m2 K

Ui=59.87w/m2 K

Designed Parameters: STRIPPER FEED BOTTOM EXCHANGER:[25-E-03]


Flow rate kg/hr 163621 159499Temperature oC 54 oC , 191 oC 242 oC , 115 oCDensity kg/m3 775 746Viscosity Cp 1.5965 0.233Sp.heat kcal/kg oC 0.5295 0.580Thermal Cond.(k) 0.098 0.091


= (191-54) / (242-54)

=0.728 (72.8%)


=(242-115) / (191-54)

=0.927


= 159499x0.582x(242-115)

= 11789209.09kj/hr

= 2816476.92kcal/hr


=163621x0.5295x(191-54)

=11869312.7kj/hr

=2835613.926kca/hr

Over all heat transfer coefficient with fouling factor 0.0004,

Udo=56.4971 w/m2K

Hi=1814.94 w/m2 oC

Ho=57.368 w/m2 oC

Operating parameters:


Flow rate kg/hr 163621 159499Temperature oC 42.2 oC, 176.6 oC 220 oC , 108.6 oCDensity kg/m3 822 702Viscosity Cp 2.707 0.357

Sp.heat kcal/kg oC 0.465 0.640Thermal cond.(k) 0.106 0.083


= (176.6-42.2) / (220-42.2)

= 0.752 (75.2%)


= 0.8288


= 163621x0.650x(220-108.6)

= 11611527.2kj/hr

= 2774028kcl/hr


= 159499x0.465x(176.6-42.2)

= 10195224.51kj/hr

= 2435669.16kcal/hr

Now, Over all heat transfer coefficient with fouling factor of 0.0004 is

Hi=1213.84w/m2 oC

Ho= 49.12w/m2 oC

Uo= 52.56w/m2 K

Ui=67.26w/m2 K

DESIGNED PARAMETERS:

COLD COMBINED FEED EXCHANGER:[25-E-04]

SHELL SIDE AND TUBE SIDE

Flow rate kg/hr 78179 90394Temperature oC 76 oC , 237 oC 270 oC , 158 oCDensity kg/m3 765.5 712.5Viscosity Cp 0.6325 0.480Sp.heat kcal/kg oC 0.5495 0.612Thermal cond.(k) 0.094 0.087


= (237-76) / (270-76)= 0.8298 (82.9%)


= (270-156) / (237-76)= 0.6956


= 90394x0.612x(270-158)= 6195966.336kj/hr= 1480234.04kcal/hr


= 78179x0.5495x(237-76)

= 6916457.04kj/hr= 1652362.04kcal/hr

Over all heat transfer coefficient with fouling factor as 0.0004,Udo= 49.50 w/m2 KHi= 9030.94w/m2 oCHo= 37.30 w/m2 oC


Flow rate kg/hr 78179 90394Temperature oC 83.1 oC , 189 oC 240 oC , 137 oCDensity kg/m3 822 674Viscosity Cp 1.895 0.309Sp.heat kcal/kg oC 0.477 0.611Thermal cond.(k) 0.104 0.08


= (189-83.1) / (240-83.1)= 0.674 (67.4%)


= 0.9726


= 90394x0.661x (240-137)= 6154294.667kj/hr= 1470279kcal/hr


= 78179x0.447x (189-83.1)= 3700782.7kj/hr= 884127.76kcal/hr

Over all heat transfer coefficient with fouling as 0.0004,Hi= 9030.15w/m2 CHo= 37.30w/m2 CUo= 36.23w/m2 KUi=45.73w/m2 K

Designed parameters:

STRIPPER FEED EFFLUENT EXCHANGER: [25-E-05]

Flow rate kg/h 163621 180787Temperature oC 191 oC , 243 oC 301 oC , 270 oCDensity kg/m3 709 662.5Viscosity Cp 0.413 0.288Sp.heat kcal/kg oC 0.6175 0.674Thermal cond.(k) 0.0865 0.078


= (243-191) / (301-191)

= 0.472 (47.2%)


= (301-270) / (243-191)

= 0.5961


= 180787x0.674x (301-270)

= 3777363.57kj/hr

= 902423.32kcal/hr


= 763621x0.6175x (243-191)

= 5253870.31kj/hr

= 1255165.1kcal/hr

Over all heat transfer coefficient with fouling factor as Rd= 0.0004 is,

Udo= 11.494 w/m2K

Hi= 11836.054 w/m2 oC

Ho= 11.457 w/m2 oC


Flow rate kg/hr 163621 18078Temperature oC 177o C, 220o C 259oC , 140oCDensity kg/m3 728 651Viscosity Cp 0.016 0.02Sp.Heat kcal/kg oC 0.594 0.688Thermal Cond.(k) 0.090 0.076


= (220-177) / (259-177)

= 0.524 (52.4%)


= (259-140) / (200-177)

= 2.767


= 18078x0668x (259-140)

= 1480082.016kj/hr

= 353595kcal/hr


= 163621x0.594x (220-177)

= 4179207kj/hr

= 998423.17kcal/hr

Over all heat transfer coefficient with fouling factor of Rd= 0.0004 as;

Hi= 11296.13w/m2 oC

Ho= 10.135w/m2 oC

Uo=11.313w/m2 K

Ui=1306824w/m2 K

FACTORS EFFECTING RATE OF HEAT TRANSFER:

The thickness of the material of the tube The temperature difference between the two inlet fluids The thermal conductivity of the material of construction The physical size of the exchangers and surface area of the tubes The number of tube side passes and shell side passes through the exchanger.(these causes

turbulent flow which increases the heat transfer efficiency by giving more residence time to the fluid in the exchanger

The type of flow, either co-current or counter current flow or the cross flow. In which the counter current flow is very effective.

The choice and design of the exchanger depends upon process requirement ex: pressure drop across the exchanger, heat transfer rate and type of fluid being used.

The temperature difference between the fluid. This is the driving force in heat exchanger principles. The greater the temperature the greater the heat transfer

Fluid flow rate increases the heat transfer rate The nature of the heat conducting materials. Some materials have a high conductivity

while others don’t. this factor is in built in the design of the exchanger and choice of material. It is governed by the design engineer before manufacturing

Surface area – the larger the surface area of the conducting interfaces, the greater the heat transfer rate

SCOPE OF THE WORK:

1. As it is known from the above points that the temperature difference between the two fluids is the driving force in heat exchanger principles and the same have been seen in the Stripper Bottom Feed Exchanger where there is a maximum difference between the two inlet fluids than the designed specifications which increases the efficiency of this 25-E-01 & 25-E-03 exchangers by η=43.5% to η=48% i.e, almost 3.5% increase in the effectiveness of this exchangers.

2. The same can be applied to Hot Combined Feed Exchanger 25-E-02 & Cold Combined Feed Exchanger 25-E-04, In which Over all heat transfer coefficient Uo= Ho and also Hi>>Ho. This shows that over all heat transfer coefficient in a heat exchanger is dominated by the smaller heat transfer coefficient when the difference between the two values (Hi>>Ho) is large. To improve the efficiency up to the designed specifications ( i.e), η=78.6% which is greater than the operating efficiency η=71.8% of 25-E-02 and η=82.% > η=67.4% of 25-E-04, we must use some enhancement technique on the tube side, such as finned surface.

3. The inside and outside film heat transfer coefficient U and Ui is differ significantly in 25-E-01, 02, 03, 04 & 05 by 10-30%. This is because of considerable difference between heat transfer surface areas on the inner and outer side of the tube. For tubes of negligible thickness, the difference between the two over all heat transfer coefficient would be negligible.

4. Some other causes of low heat transfer is because of Fouling , deposits of scale, dirt sand, and/or other solid deposits on conducting surface. Coke formation in furnace tubes and other causes of semi-blockage of tubes will drastically decrease efficiency in exchangers. Such problems will leads to shut down for cleaning and possible replacement. Many of these problems can be avoided by proper operation and fluid treatment, filtration, corrosion inhibitors, furnace firing control etc.

5. The material used in the construction of heat exchanger may be an important consideration in the selection of heat exchanger. A temperature difference of 50oC or more between the tubes and the shell side will probably pose differential thermal expansion problems and need to be considered. In case of corrosive fluid, we may have to

select expensive corrosion resistant materials such as stainless steel or even titanium if we are not willing to replace low cost heat exchangers frequently.

Efficiency of Heat Exchangers

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