http://maintenanceengineering.in/HEAT

%20EXCHANGERS,STRAINERS.htm

HEAT EXCHANGERS, STRAINERS AND SEPERATORS

 

HEAT EXCHANGERS:

A heat exchanger is a device for transferring heat from one fluid to

another, such that fluids are kept separated. They are widely used in

refrigeration, air-conditioning, industrial heating, power production or

chemical and petrochemical industries in various processing. A radiator in

a car is a heat exchanger in which hot radiator fluid is cooled by the flow

of air over the radiator surface.

          Broadly heat exchangers may be classified according

A) Their make type

1)     Shell & Tube type

2)     Plate type

3)     Finned tube type

B) flow arrangements:

1)     Parallel flow,

2)     Counter flow or cross flow,

C) type of heat exchangers tubes

1)     straight tube

2)      U-tube

D) Type of tube sheets:

1)     floating tube type

2)     fixed tube and

E) Number of passes

1)     one pass

2)      Multy-pass

F) Flow of liquids

1)     Single flow

2)      Split flow or divided flow.

 

The two fluid in heat exchanger may be both liquid or one liquid and

another gas or both gas. For better efficiency heat exchangers are

designed to maximize the surface area between the two fluids, contact

period of two fluid while minimizing resistance to their flow. A typical heat

exchanger is the shell and tube heat exchanger in which one fluid flow in

the tube and the other fluid flows in the shell.

          A heat exchanger may be used to cool the discharge gas of

compressor as an interstage cooler or to produce steam by the waste heat

of flue gas of process as a waste heat boiler. Condenser or cooler are

basically heat exchangers but name has been given as per the service.

          The international standards for Heat Exchangers is TEMA (Tubular

Exchanger Manufacturer Association) which define the type of the heat

exchangers, design factor, fabrication etc.

          In General a heat exchanger consists of following parts:

i)                   Shell

ii)                 Tubes or Tube bundle

iii)               Tube sheets 

iv)               Baffle Plates

v)                 Tie-rods 

vi)               Inlet Nozzle 

vii)             Outlet Nozzle 

viii)           End channel covers “Plenums”

ix)               Floating head etc.

         

 

 

TEMA NOMENECLATURE:

It is recommended by TEMA that heat exchanger size and type be

designated by number and letters. And shall be a part of equipment name

plate. TEMA nomenclature represents not only the type of heat exchanger

but also give size of exchangers. e.g. dia of the tube sheet, length of

tubes etc. Examples of TEMA nomenclature:

SIZE 23-192 TYPE AES means split ring floating head exchangers with

removel channel and cover ,single pass shell size 23” inside diameter with

192” long tube.

SIZE 17-192 TYPE CEN means fixed tube heat exchanger having stationery

and rear heads integral with tube sheets, Single pass shell, 17” inside

diameter with 192 inch (16’, ft) tube length

SIZE 19-84 TYPE BGV mean U- tube exchanger with bonnet  type

stationery head split flow shell 19” inside dia with 84” tube length.

The material of the heat exchanger  may be carbon steel, stainless steel

or alloy steel and the material of the tubes may be carbon steel alloy

steel, stainless steel, copper etc. depending up on the service condition

requirement. Heat exchanger design depend upon fluid parameters

(corrosive or non corrosive) temperature, pressure, or phase of fluid etc.

There are certain compact design which may be used where area

constrain is observed. Heat exchanger tubes are of different types. For better efficiency, finned

tubes can be used. Finning of tubes also help in making heat exchangers

compact. Finning may be inside of the tube or out side or both side,

depending on the service requirement.

          During fabrication, all NDT are followed as per QAPs but as a safety

measure and reliability improvement, all heat exchangers are

hydrostatically tested in accordance with ASME code.

The TEMA standards for Class “R’ heat exchangers  specify design,

fabrication and materials standard of un-fired shell and tube heat

exchangers for the general service requirement of petrochemical and

related processing application . Safety and durability are also covered

under this standard. Heat exchangers shall comply with the ASME, Boiler

and pressure vessel code, section VIII Div. I.

FOULING :- During service , slight deposition of coating on the tube

greatly reduces  its efficiency and this is indicated by pressure drop or

reduction in performance. The deposition of un-wanted coating is called

fouling. Fouling takes place on the path of flow of one of the fluids. It is a

common phenomenon in heat exchangers. Exchanger subject to fouling or

scaling should be cleaned periodically depending on the service

conditions. Air or vapor binding also reduces the performance. Air or vapor

binding happened when foaming fluids flows through the exchangers. For

the purpose, depending on the service conditions fouling factors are taken

in to consideration while designing the heat exchangers.

Generally tubes are cleaned by mechanical methods or by chemical

treatment depending upon the nature of scaling and accessibility. High

pressure hydrojetting is the common procedure of de-scalling of industrial

heat exchangers.

LEAKAGES :- Heat exchangers in services poses problems in form of

leakages of fluids and thus two fluids get mixed. Leakages are of two

types.

1)     Tube to tube sheet joint leakage

2)     Tube leakage

Leaking of tubes is a common phenomenon and this is indicated by the

mixing of two fluid in service and also reduction in performance. e.g. in an

oil cooler with water in the tubes and oil is circulated in the shell at higher

pressure than cooling water in the tubes and if any tube leak then trace of

oil will be found in the cooling water or vice versa. Similarly in CO2 gas

cooler with gas on tube side and the pressure in the tube is greater than

cooling water in the shell and if tubes leak then CO2 gas will enter in the

cooling water which in turns will reduce the Ph of the water.

The solution to this problem is either the leaky tubes should be replaced

which is very difficult in a process plant or the leak tube may be isolated

by plugging from both the ends.

In general there is margin of approx 10% of total tubes in the heat

exchangers which can be plugged if required during the service. When

plugging of tubes exceeds this numbers tube bundles are replaced.

Another problem associated with heat exchangers is the corrosion of

channel covers on cooling water side. Galvanic corrosion is taken care by

providing the sacrificial anodic metal coupons in the channel covers.

Corrosion is taken care by the protective coating of suitable paint.

STRAINERS : Strainers are device which helps in arresting or restricting

flow of unwanted foreign particles like pipeline debris or seal/jointing

compound, weld metal,scaling  and other solids in flowing liquids or gases,

which may damage the down stream equipment or reduce the efficiency.

A pump or compressor shall have suction strainers so that clean fluid

enters into the system. A strainer should be fitted at upstream of every

steam trap, flow meter and control valve to avoid malfunctioning.

Strainers can be classified according to their body configuration or shape:

e.g.

1)     Y-type

2)     Basket type or “Tee” type

3)     Bucket type

4)     Conical etc.

Y- TYPE STRAINERS:

These look as if horizontal Y shape fitting has been installed in the

pipeline. These strainers have lower dirt holding capacity than basket type

strainer. On application where significant amount of debris are expected a

blow down valve can usually be fitted in the strainer cap, which enables

the strainers to use the pressure of the fluid to be cleaned and without

having the system shutdown. Y-type strainer in horizontal steam or gas

lines should be installed in such a manner so that the pocket is in the

horizontal plane. This stops water collecting in the pocket helping to

prevent water droplets being carried over which can cause erosion and

affect heat transfer processes.

On liquid system however the pocket should point vertically down wards,

this ensures that the removed debris is not drawn back into the upstream

pipeline during low flow conditions. Installation of Y-type strainer is not

possible in case of vertical line upward flow but in vertical line down ward

flow it is possible and very effective.

 

 

BASKET TYPE STRAINER:-

These strainers are more suitable where flow is very high. These can be

installed in horizontal pipe line or vertical line in down ward flow only. In

case of steam line if basket or bucket type strainer are installed, a

significant amount of condensate may be formed and which shall be

removed by drain plug or steam trap. Basket type strainer has a greater

free straining area and also the pressure drop across the strainer is less

then Y-type strainer.

BUCKET TYPE Strainers:

Bucket type stainers are more suitable for thin or low viscocity fluids or

gases. They provide higher straining areas than any other type of

strainers. These strainers can be installed in horizontal lines only. Rate of

increase of pressure drop is normally very slow as compares to conical

strainers.

CONICAL STRAINERS:

These are conical in shape and can be installed in either direction, over

the cone or under the cone. These strainers can be installed in any

pipelines and are preferred in case of gases where flow is very high.

Because they helps in stream lining the inlet flow to any machine.

Disadvantage is that, a spool piece is required to be opened for cleaning

of this type of strainers.

 

Basically a strainer consists of a screen, which screened the fluid. There

Screens are of three following types:

a) PERFORATED SCREEN :-These are screen with a large number of

holes in a flat sheet of required material and of required hole size. The

perforated sheet is them rolled or fabricated into required profile. 

Mesh Screen:

Fine wire is formed into a grid or mesh arrangement. This is then commonly layered over a perforated screen, which acts as a support cage for the mesh. By using a mesh screen, it is possible to produce much smaller hole sizes than with perforated screens. Hole sizes as small as 0.07 mm are achievable. Subsequently, they are used to remove smaller particles which would otherwise pass through a perforated screen. Mesh screens are usually specified in terms of 'mesh'; which represents the number of openings per linear inch of screen, measured from the centre line of the wire. Figure shows a 3 mesh screen

The corresponding hole size in the mesh screen is determined from

knowledge of the wire diameter and the mesh size; it is usually specified

by the manufacturer. The maximum particle size that will be allowed to

pass through the screen can be determined using geometry. If, for

example, a 200 mesh screen is specified and the manufacturer's

specifications stated that the hole size is 0.076 mm, then the maximum

particle size that will pass through the screen can be found using

Pythagoras' theorem: where C = maximum particle size

C2   =  A2 + B2

 

 

 

b) WOVEN SCREEN :- Mesh or Grid is formed by woven wires.

 

c) NOTCH WIRE SCREEN:-

The notch wire element is manufactured by wrapping specially treated

thin stainless steel wire(special cross section wire called notch wire)

around the cylindrical filter frame. It has a simple structure and robust

form, allowing very accurate setting of the filtering passage size. In

addition, because the element is not subject to corrosion or deterioration,

and impurities adhered to the element can easily be eliminated by

backwashing or air blowing, the element requires no replacement and has

a semi-permanent service life. Because of its high reliability . Notch wire

mesh provides more area of opening per unit area of the mesh as

compared to woven mesh

FILTER :- Very fine strainers are called filters. Strainer are generally used

in process fluids just before the rotary machines  while Filter remove very

fine particles from the fluid and are used in lubrication system and or fuel

gas and instrument air system. Generally a strainer is installed in the

suction of a pump or compressor where as filter are installed in discharge

of the pumps of lubrication system etc.  

SEPARATORS: -

To separate two different fluids from each other while in flow and thus to

move unwanted or damaging suspended fluid from gas stream separator

are used. In case of steam water droplets are separated from steam , in

case of oil mixed gas oil is separated from the gas or condensate is

separated from the gas. Separator are must required to separate out

these suspended fluid from the gaseous stream as these fluid contents

may damage the equipment/system (condensate in case of compressor)

or may be re-used later (incase of oil which is reused again). In process

also it become desirable to separate out oil from the gas as oil may

damage the catalyst.

These separators are basically liquid traps.

In case of steam there is always water droplets, which eventually

gravitate towards the bottom of the pipe, which affect the system as

below:

 

-        Water droplets works as an extremely effective barrier for heat

transfer .

-        Water droplets traveling at high velocity will erode the pipe , valve

seat or fittings and also increase the corrosion rate if any.

-        Increased volume of condensate may cause water hammer, a

deadly process for any piping.

-        Increased of scaling.

-        May damage the rotating part like steam turbines etc.

         

There are basically three type of separator

 

i)BAFFLE TYPE :-  In case of baffle type separator number of baffle plates

are there which change or reverses the direction of flow of the main fluid

in the separator body. Thus the greater mass (water droplets in case of

steam or air and oil in case of gas) fluid collects on the baffles, which is

then collected in the bottom and drained away.There is a resulting

reduction in the speed of the fluid.This reduces kinetic energy of the

droplets and most of them will fall out of suspension.In case of steam

condensate will collects at the bottom of the separator which will be

drained away through steam trap.

ii)CYCLONIC TYPE : The cyclonic or centrifugal type separator uses a

series of fins to generate high speed cyclonic flow which throw the heavier

suspended fluid to the wall of the separator which is then collected in the

bottom and drained.

 

iii)COALESCENCE TYPE :- This type of separator provide an obstruction

by wire mesh pad which is called demister pad upon which suspended

fluid droplets get entrapped. These suspended fluid particles tend to

coalesce, producing droplets that are too large to be carried further by the

gas. As the size of the droplets increases they become too heavy and

ultimately fall into the bottom of the separator.

The pressure drop across a baffle type separator is very low due to the

reduction in the velocity, which is created by the large increase in cross

sectional area provided by the separator body. The pressure drop across a

cyclonic type separator is somewhat higher, as the velocity of the fluid

has to be maintained to generate the cyclonic effect.

         

The selection of separator depends upon the velocity of the fluid, size of the pipeline, pressure drop allowable and the type of the fluid. By proper selection of separator maximum efficiency can be achieved. Operating pressure and flow rate of the fluid are also a criteria for selection of separators.