Heat Exchangers Flashcards
Why is understanding the heat exchanger process so important?
Many processes use dangerous fluids(toxic, flammable or termperature extreme)
Depending on The fluids in the exchanger, ________ and purging may be required and valves pumps or compressors will need to be_______
Venting
Locked out
What are the different types of matter used in a heat exchangers to cool the material?
Gas to gas
Liquid to gas
Liquid to liquid
Gas to gas heat exchange is typically used with what ________
Compressors
As compressors get into higher compression ratios or higher flow the heat exchange is ______to gas method
Liquid - glycol or water is common to cool compressed gas
Liquid to liquid is very common heat exchange method in the industry. What is the main difference?
Both sides of the tubes has liquid
What are the three means of heat transfer?
Conduction
Convection
And radiation
Which is the most important forms of heat transfer used in heat exchangers?
Conduction
Why is copper typically used in heat exchangers?
Has a high thermal conductivity rating
All metals are better thermal conductors than liquids, liquids are better conductors than______.
Gas
Conductivity of silver is 750 times that of water
Water conducts heat 25 times better than air
Convection occurs when?
Movement of a mass of fluid from a warm location to a cooler one.
Central heating is a good example of that
Convection currents are often increased through the use of auxiliary equipment such as? This is called ________\
Pumps or fans are used to move mass
This is called forced convection
Heat exchangers use both conduction and ________ methods of heat transfer.
Convection
The last type of heat transfer? Minor part in heat exchangers.
Radiation
What are the different flow path arrangements in heat exchangers?
Parallel flow
Counter flow
Cross flow
Mixed flow
Parallel flow
Fluid flow in same direction
Counter flow
Much more efficient than parallel flow.
Cross flow
More efficient that parallel but not as efficient as counter. Advantage is able to produce more turbulence, which makes for greater efficiency for given size exchanger.
Mixed flow
Uses baffles to cause flow to switch between parallel and counter flow
What are the different designs of heat exchanger?
Finned tube
Plate
Spiral
Cooling towers - natural draft
- mechanical draft
Double pipe
Shell and tube
Finned tube
Most common
Car radiator
Plate heat exchanger
A number of pressed plates clamped together on a frame with gaskets between the plates.
Spacing is between 2-5 mm
Advantages of plate heat exchangers
- Low fouling
- high heat transfer
- little floor space required
- little clearance needed
- no insulation required
- little vibration
Disadvantages of plate heat exchangers
- limited by pressure and temperature
2.07 kPa and 175degree C - not recommended for viscous (thick) fluids or fluids that change viscosity with heat
- small clearance between plays limits the size of solids passing though
- high pressure drops not suitable for air coolers and gas to gas services
Spiral heat exchanger
Very efficient do to both fluids having a true counter flow path. The single path for each side eliminates dead bypass areas which lessons fouling.
Continually curving passage creates high turbulence
What is the purpose of a cooling tower?
Recycle cooling water for large scale applications such as air conditions and power generation.
What is the general principle of a cooling tower?
Water to be cooled is pumped to the top of the tower where it is sprayed through nozzles to give a fine spray
Efficiency of a cooling tower depends on?
- area of water (smaller droplets give larger surface area)
- length of contact time between air and water
- relative humidity
What are the two classes of cooling towers?
Natural draft and
Mechanical draft
Natural draft cooling towers depend on ________ operate effectively in _________ with fairly steady wind. Maintenance is fairly simple since limited moving parts
Wind conditions
Large open areas
Mechanical draft cooling towers use _______ to increase the air flow though the exchanger and are divided into two classes: forced draft and _______-
Fans
Induced draft
Forced draft towers
Use fan to push air through the tower
- little vibration or noise
- air is fairly dry which reduces problems with erosion, corrosion and scale build up on the fans
What recaptures most of the water that is being pulled out with the air flow in a mechanical draft tower
Drift eliminators
What is the difference between induced forced draft cooling towers?
Induced tower has fan on top pulling air up through the water as it flows down through the packing.
Venturi towers can either forced or ______
Induced
Maintenance required on cooling towers consist of :
- lubrication of fan motor every 3 months
- inspect fan belt regularly
- inspect bearings on floating shaft assembly
- ensure float valve operates freely
- inspect structural integrity yearly
- clean and inspect fan blades
- consult water treatment experts if excessive scaling or algae is present
Double pipe exchanger
Compact and components and fittings are small in dimension, which makes natural choice for high pressure services
Does not provide large heat transfer surface areas and is commonly used for low heat duties with smaller surface area requirements
When larger heat exchange is needed multiple units can be hooked together
Advantages of double pipe heat exchangers?
- units can be added or removed as needed
- modular Type construction simplifies maintenance
- suitable for high pressure application
- can be grouped together in parallel or in series for the required cooling
Disadvantages of double pipe heat exchangers
Limited to low heat duties requiring smaller surface areas
Most common type of heat exchanger?
Shel and tube
Capable of high temps and high pressure
Heat exchangers are typically made of what materials
Non corrosive:
- copper
- brass
- stainless
- Carbon steels
Why would a anorode be used in a heat exchanger?
Anarods are used as sacrificial replaceable parts that take the electrolytic actions instead of the heat exchanger itself
Longer the heat exchanger tubes means _________ the number of tubes and holes necessary and a smaller shell diameter can be used. Disadvantage of longer tubes is _______ pressure drops
Lower
Higher
The greater the surface area of the tubing the quicker the heat is _____. _______ is one of the more common tube materials due to its high thermal conductivity
Conducted away
Copper
Welded tube joints are commonly used in _________ pressure or extreme temperature applications where considerable expansion must be handled
High
Tube sheet layout
30 degree triangle pitch greatest heat transfer rate but greater pressure drop across the exchanger
What two functions do baffles do?
Support tubes to prevent tube vibration and sagging
Cause turbulence to increase the heat transfer rate and prevent dead airspace areas
What are the different designs of the baffles?
Segmental - most common
Disc and doughnut - smooth flow can cause sedimmate buildup of fluids are not clean
Impingement baffles - provided over the tubes to reduce corrosion by the entering fluid
Longditudinal baffles - split the shell Side flow into two or more passes allowing more contact time between fluids
Single pass tube side/single pass shell side -
What are shell and tube exchangers?
Cylindrical outer shell surrounding a bundle of tubes that are parallel to the longitude axis of the shell.
The principle types of shell and tube exchangers are fixed tube sheet, u-tube and _______
Floating head
Fixed tubesheet
Tube sheets are welded to the shell at both ends. Tubes are straight and can almost completely fill the shell.
- no gasket joints so it is ideal for high pressure and dangerous fluid
What is the main disadvantage of a fixed tubesheet heat exchanger?
Tube bundle cannot be removed for cleaning. Best suited when fluids are clean
Advantage of a u-tube heat exchanger?
Handles differential thermal expansion
- u-tube bundle can easily be removed
- vibration and erosion damage can be avoided by locating nozzles at a point beyond the u-bends
- well suited for high pressure high temperature
What are the different types of floating head shell tube exchangers?
Floating head shell
Floating head with split backing ring exchanger
Floating head, outside-packed, lantern ring exchanger
Double tube sheet exchanger
Advantage and disadvantage of a floating head shell tube exchanger?
Can handle extremely high fluid temperature differential since the tubes can expand and contract without stress.
Disadvantage: large clearance requirements needed between the outermost tubes and the inside of the shell
Advantages of the floating head and split backing ring exchanger
Widely used in oil and gas. Backing ring allows for greater number of tubes to fit inside the same diameter shell. Greatly increases the efficiency for a exchanger this size.
What is the disadvantage of the outside packed lantern ring exchanger?
Can leak so hazardous toxic, lethal or explosive chemicals should not be used with it
What is the use of a double tubesheet exchanger.
Applications. Where it is not desirable to have mixing of the tube side and shell side fluids
Only option if mixing of two fluids must be prevented.
How do you decide which fluid goes through the tubes and which fluid goes through the shell of an exchanger?
No hard and fast rule but some things to consider are:
- dirtier fluid should go through tubes since they are easier to clean
- higher pressure fluid should go through tubes since they can collapse
- corrosive fluids should go through the tubes or tube and shells will corrode
- fluid with lower heat transfer coefficient usually travels on the outside of the tubes
What are the major factors that determine shell and tube exchangers efficiency?
Tube length and shell diameter
What factors affect heat exchangers efficiency
Fluid flow
- temperature differentials
- heating surface area
- contact time
- conductivity of the metal wall and nature of the fluids
- fluid films adjacent to the inside outside or metal wall surfaces
-Thickness of the heating surface
- cleanliness of the heating surface
Increased velocity of fluids results in!
Turbulent motion and therefore a better rate of heat transfer is obtained
Thick wall tubes require a high conductance metal to transfer the heat rapidly and efficiently what types of metals are typically used?
Copper
Copper alloys
Brass
Steel
Nickel aluminum
Low carbon steels
What are some factors when trouble shooting heat exchangers?
- performance criteria (flow rates, thermal design, operating pressures and temps)
- proper pressure and temperatures
- fouling characteristics, frequency and quality of cleaning
- proper handling and storage
- proper design, base supports and piping
- materials, tools and workmanship
Poor heat exchanger performance can be traced to one of the following:
- dirty exchanger
- failure to remove preservation materials after storage
- operating conditions different than design conditions
- corrosion from incorrect application of exchanger materials or conditions
- air or gas build up
- leaking tube sheets or gaskets
- excessive clearance causing dead bypass areas
- tube failure cause by erosion or vibration
- incorrect piping connections
Fouling results in?
Increase of resistance to both fluid flow and heat transfer
An increase in pressure drop through the exchanger or drop heat exchange efficiency usually indicates that cleaning is needed
What are the 3 most common problems found in exchangers?
Leaking tube sheets, fouling, and vibration
What should steam never be used to clean exchanger tubes and why?
Steam
Will overheat resulting in severe expansion strain and leaking tube sheet joints
What is one way to clean the insides of the tubes?
Brushes can be simply pushed through or rotated on a flexible shaft with a rotary tool
When removing tube bundles don’t lift ________
Never wrap slings around the tubes!
Never drag bundles
Support by end baffles
How do you pull the bundle out from the shell?
Through bolts can be inserted through the tubes and bolted to a heavy bearing plate and pulled out
When bolting tube sheets you need to do what?
Torque to specific torque slowly and follow the bolting pattern
Where do most tube failure occurs
Where tube spans are long and unsupported.
I let baffle area is a prime candidate for failure because of the span length and high local velocity
For the most accurate leak detections what would you use?
Hydro testing
1 1/2 times the working pressure
What must be done before loosening head studs?
Check for leaks
Ensure has been purged if necessary
Vented
Drained
Pressurizing a fixed tubesheet for a leak test with hot water could?
Result in enough expansion to seal the leak.
Always use cold water
