Heat Exchanger Flashcards
What are two methods of removing heat in an gas to liquid heat exchanger
Conduction, convection
Describe a liquid to liquid heat exchanger
Both sides of the tubes have a liquid flowing it in a shell and tube cooler. It is common in many industrial applications
What are some different types of fluid used in cooling process?
Gas to gas
Liquid to gas
Liquid to liquid
What are the three basic means of heat transfer?
Conduction
Convection
Radiation
What is conduction?
The transfer of heat from one source to another through direct contact
What things affect the rate of heat transfer by conduction?
Difference of temperature
Area of contact
Conductivity of the metal wall
Nature of the substance
Define convection.
It is a result of the movement of a gas or liquid, from a warm location to a cooler one
Describe an air convection cycle
Air is heated it expands and become lighter, moving upward, then heavier, cooler air falls, and takes its place to be heated
How are convection currents increased?
By auxiliary equipment such as pumps or fans. This is called forced convection
What changes the rate of heat transfer in a convection transfer?
The velocity of the fluids inside and outside the tubes
Direct contact or scrubbing action
Why is turbulent flow important?
It allows the hot and cold fluids to scrub the walls of the tubing or plates,to increase heat transfer rate
What is radiation?
Electromagnetic waves moving across space and requires no medium of transport and can travel across a vacuum
What are the common flow path arrangements?
Parallel flow
Counter flow
Cross flow
Mixed flow
Describe a parallel flow
Both fluids are moving in the same direction and there is one shell pass and one tube pass
(this is the least efficient)
Describe counter flow
The flow is in the opposite directions.
this is the most efficient
Describe cross flow
The flows are perpendicular to each other.
NOT as efficient as counter flow, but has the benefit of producing more turbulence which makes for greater efficiency
Describe mixed flow
Mixed flow is common in heat exchangers with baffles that cause the flows to switch between parallel and counter flow as they progress through out the exchanger
What two flow paths are commonly combined for greater efficiency?
Counter flow
Cross flow
What is the most common heat exchanger?
Finned tube heat exchanger
For what applications is the finned tube heat exchanger use?
Intercoolers
Aftercoolers
Stationary engine cooling
Where can you find fins on heat exchangers?
On air cooled compressors fins are common, but they can be added to the tubes inside the shell and tubes heat exchangers to increase efficiency
What is PHE?
Plate heat exchanger
Describe the Plate heat exchanger
Is a number of pressed metal plates clamped together on a frame with gaskets between the plates.
Hot and cold fluids run in opposites directions.
What is the spacing on a plate heat exchanger?
2-5 mm gap
Give some advantages of plate heat exchangers
Low fouling High heat transfers coefficients Compact No insulation required Vibration free
What is a major disadvantage of the plate heat exchanger?
Limited to temperatures and pressures, high pressure drops
Describe a spiral heat exchanger
It has a long path permitting plenty of exchange heat while keeping radiation losses to minimum. Spiral heat exchanger is very efficient due to both fluids having a true counter flow path
How much surface area of cooling can be provided by a 1 meter diameter X 1.5 meter long spiral heat exchanger?
100 meters
What are cooling towers commonly used for?
To recycle cooling water for large scale applications like air conditioning and power generation
What is the principle of a cooling tower?
The water to be cooled is pumped to the top of the tower, where it is sprayed through nozzles to give a fine spray.
What does the efficiency of the cooling tower depend on?
Area of water surface (small droplets give larger surface area)
Length of contact line
Relative humidity
Cooling towers are divided into two classes what are they?
Natural draft
Mechanical draft
Describe the natural draft cooling tower
Air movement through the tower is dependent on wind conditions. They operate effectively in large open areas with a steady wind.
Describe maintenance on a natural draft cooling tower
Maintenance is fairly simple, routine checks includes removing algae, scale and dirt buildup ensuring that the float control valve operates properly
High humidity decreases cooling efficiency
Describe mechanical draft cooling towers
They use fans to increase the airflow through the heat exchanger
What are the two styles of mechanical draft cooling towers?
Forced draft towers
Induced draft towers
Which cooling tower is the most efficient?
Induced draft cooling tower (true counter flow)
What is a Venturi cooling tower?
It can be either forced or induced, but has no fan of its own. It uses the airflow from the system or process
What is a disadvantage of Venturi cooling tower?
It has a parallel flow which is least efficient and it will contain airborne contamination
What are some advantages of Venturi cooling tower?
It is quiet operation
Minimal maintenance
No electrical connections
What is a double pipe heat exchanger?
Simplest form is a pipe inside a pipe. It is excellent for high pressure service. But does not provide large heat transfer surface areas, so it’s more suited to low heat duties.
What is the most widely used double pipe heat exchanger?
The hair pin style heat exchanger, because it handles high stresses produced by two pipes difference thermal expansion.
What is the most common type of heat exchanger found in the Millwright trade?
The shell and tube heat exchanger.
Why is sacrificial zinc anode commonly mounted on heat exchangers?
Because zinc anode gets eaten by electrolytic anode action.
What is the main disadvantage of longer tubes?
Longer tubes have a greater surface area but they also have a great pressure drop due to it’s surface area of it’s longer tubes.
What is the most common material tubes are made from? And Why?
Copper because it has high thermal conductivity.
How are tubes connected to the tube sheet?
Rolled Tube Joints/Expanded
Double Tube Joints/Expanded
Welded Tube Joints
What is a tube bundle?
Tubes and tube sheets packaged together.
What is an advantage and a disadvantage for cleaning and repair of a U-tube heat exchanger?
The advantage and disadvantage are that the two bundles can be completely removed for repairs and cleaning. However, the radius at the end leads to an increase of wear at the bends.
Describe a gas heat exchanger?
In the gas heat exchanger the hot compressed gas travels around the tubes, while the cooler water/glycol travels in the tubes of the heat exchanger.
What are the two square types of heat exchangers?
90° and 45°
What are the two triangular types of heat exchangers?
30° and 60°
What is the purpose of the baffle?
To support the tubes and to cause turbulence flow to increase heat transfer rate and prevent dead bypass area.
What are the two types of baffles?
Segmental and Disc and doughnut baffles.
What are two methods of removing heat in an air to liquid heat exchangers?
Conduction and Convection.
What is an impingement baffle?
It’s a baffle that sits perpendicular to the shell side inlet, protecting the tubes from erosion by entering high-pressure fluids.
What is a longitudinal baffle use for?
Its used to split the shell side flow into two or more passages.
I you have three (3) longitudinal baffles, how many passes does it get?
4 Passes.
(Remember for every longitudinal baffle you get one more extra pass) e.g. 1 baffle 2 passes
How are the number of tube side passes determined?
By the number or baffles in the channel head.
What are the different types of shell and tube exchangers?
Fixed tubesheet, U-tube Floating head Floating head with split backing ring Floating head outside packed lantern ring Double tubesheet
What is a floating head shell and tube exchanger?
One tube sheet is bolted onto the flange and the other end is free to float. This permits the exchanger to use high temperature differentials because it can expand and contract without stresses.
What is a fixed tube sheet shell and tube?
The tubesheet is welded to the show at both ends. It cannot be cleaned and is bad for high temperature because it has no room to expand. Also the tube sheet has no gasket joints which makes it good for high pressure, or dangerous fluid applications.
Describe the U-tube type shell and tube exchanger
Both ends of the U shaped tube are fixed to a single tubesheet, because of this thermal expansion is not a problem however you can get wear and erosion on the end of the bend.
What is a great advantage of the u-tube exchanger?
Because of the fixed tubesheet it eliminates the problems of differential thermal expansion
What is an advantage of the floating head exchanger?
The tube bundle can be remove for shell side cleaning without removing the shell or floating head covers
What is a disadvantage of the floating head exchanger?
The large clearance requirements needed between the outermost tubes and the inside of the shell require for gasket and bolting of the floating head cover to the tubesheet
What is the floating head with split backing ring?
The shell and tube fluids are separated at the floating head by fitting the floating tubesheet with a flanged and gasketed floating head cover and holding it in place by bolting it to a split backing ring on the other side of tubesheet.
What is the order of disassembly of a split backing ring floating head exchanger?
- shell head
- floating head cover
- split ring
- shell
- channel head
What is a floating head, outside packed, lantern ring exchanger?
It uses a lantern ring around the floating tubesheet to separate the two fluids. Leaks are detected by fluids dripping through the weep holes, therefore it cannot be used with dangerous fluids.
What is a double tubesheet exchanger?
It is used when it is crtical the two liquids do not contact each other. It is impossible for fluid to fluid to leak from the tubesheet, however a tube could possibly rupture.
Which fluid goes through the tube and which fluid goes through the shell?
Dirtier fluid, high pressure fluid, corrosive fluid goes through the tube
Cooler fluid goes through the shell
What group specifies and standardizes exchanges sizes and parts?
TEMA , Tubular Exchanger Manufacturers Association
How do you size a tube and shell exchanger
shell diameter / tube length
24” to 197”
is 24” diameter shell , 197” tube lenght
What are some things that slow down the heat transfer efficiency?
Thickness of the metal wall will influence the transfer, deposits or skill and dirt on the tubes would have low conductivity and slow the heat flow rate
How is heat transferred in a shell and tube heat exchanger?
Conduction
What is the main problem with a fixed tubesheet?
Chemical wash is the only way to clean the shell side
Whys is copper commonly used in heat exchanger?
It has good thermal conductivity
What should not be used to clean the tubes on a heat exchanger?
Steam
Segment baffles are used to
Dampen out vibrations
Support the tube bundle
Increase contact time
Prevent dead bypass areas
A heat exchanger using corrosive fluid should have it flow through which?
The tubes
Describe a gas to gas heat exchanger
Commonly used in compressor operations with intercoolers and aftercoolers
Why does water have the ability to absorb or reject more heat than air?
Because it is more dense than air
Which heat exchanger is very efficient due to both fluids having a true counter flow?
Spiral heat exchangers
