Heat Exchanger Flashcards
Describe a gas to gas heat exchanger
Commonly used in compressor operations with intercoolers and aftercoolers. The hot compressed gas runs through the tubes with the cooler atmospheric air simply being blown across the outside fins to supply the necessary cooling
Why does water have the ability to absorb or reject more heat than air?
Because it is more dense than air
Describe a liquid to gas heat exchanger
The hot compressed gas travels around the outside of the tubes while the cooler water or glycol travels through the tubes in the shell and tube heat exchanger
What two methods of removing heat in an air to liquid heat exchanger
Conduction, convection
Describe a liquid to liquid heat exchanger
Both sides of the tubes have a liquid flowing through it in a shell and tube cooler. It is common in many industrial applications
What are some different types of fluid used in cooling processes
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 becomes lighter, moving upward. The heavier, cooler air falls, and takes its place to be heated
How are convection currents increased?
Auxiliary equipment such as pumps or fans. This is called forced convection
What things change the rate of heat transfer in a convection transfer?
The velocity of the fluids inside and outside the tubes is a major factor. The amount of direct contact or scrubbing action also affects
Why is turbulent flow promoted?
It is built into heat exchangers to allow the hot and cold fluids to scrub the walls of the tubing or plates to increase heat transfer rate
What is radiation heat transfer
It requires no medium of transport, and travels across a vacuum. The sun is an example of radiant heat
Which type of heat transfer plays a small part in heat exchangers?
Radiation heat transfer
What are the common flow path arrangements?
Parallel flow, counter flow, cross flow, mixed flow.
Describe a parallel flow path
Both fluids are moving in the same direction, and there is one shell pass, and one tube pass
This is the most inefficient.
Describe counter flow path.
The flow is in the opposite directions. This is the most efficient transfer method
Describe cross flow path
The fluids are introduced perpendicular to each other. Not as efficient as counter flow, but has the benefit of producing more turbulence which makes for greater efficiency.
What two flow paths are commonly combined for greater efficiency?
Counter flow and cross flow
What is a mixed flow path?
Half the time is parallel flow and half the time it’s counter flow
What is the most common heat exchanger?
The finned tube heat exchanger. It is the same style as on a car radiator
What applications would use the finned tube heat exchanger
It would be used on compressor intercoolers and aftercoolers and stationary engine cooling
Where can you find fins on heat exchangers
They are common on the outside of air cooled compressors, but fins can also be added to the tubes inside shell and tube heat exchangers
What is a PHE?
Plate heat exchanger
Describe plate heat exchangers
It is many pressed plates clamped together in a frame. The plates is corrugated with alternating ridges and grooves. The pattern gives the flow a turbulent path, the hot and cold fluids run in opposite directions in the spaces between the plates
How is the spacing on a plate heat exchanger?
Spacing is very important. They are placed between 2-5 mm gaps
Give some advantages of plate heat exchangers
They are low fouling, they have high heat transfer coefficients, they are compact, The reading can be changed easily by burying the number of plates, no insulation required, vibration free
What is another benefit of a plate heat exchanger
They are good for fluids with high turbidity
What is a major disadvantage of the plate heat exchanger
It cannot run with high-pressure
What is a very important factor with the plate heat exchanger
The flow rate is critical
Describe a spiral heat exchanger
It is very efficient due to both fluids having a true counterflow path. The continual curving passages creates high turbulence. It has a long path, so there is much heat transfer opportunity.
How much surface area of cooling can be provided by a 1 meter diameterX1.5 meter long spiral heat exchanger?
100 square 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 time
Relative humidity
Cooling towers are divided into two classes what are they?
Natural draft, mechanical draft
Describe a natural draft cooling tower
Air movement through the tower is dependant on wind conditions. They operate effectively in large open areas with a steady wind.
Describe maintenance on a natural draft cooling tower
Mtnc is simple because there are very few moving parts. Worn or dirty nozzles can cause problems, and on excessively windy days there could be high loss of water. High humidity levels decrease the cooling efficiency of this cooler
Describe mechanical draft cooling towers
They use fans to increase the airflow through the exchanger
What are the two styles of mechanical draft cooling towers?
Forced draft towers, and induced draft towers
Which cooling tower is the most efficient?
The induced draft cooling tower, as it is 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 downfall and advantages of Venturi cooling tower?
It has a parallel flow which is least efficient, and will contain airborne contamination. It is quiet operation, minimal maintenance, and 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 hairpin style because it more easily handles the high stresses produced by different thermal expansion of the two pipes.
What is the most common type of heat exchanger found in industry.
The shell and tube heat exchanger. It is well suited for pressure applications and high temperature because it posses larger surface areas for heat transfer
Why is A sacrificial zinc anode commonly mounted in heat exchangers?
Zinc anode is eaten away by electrolytic action. Electrolytic action is caused by contact between different metals. Once the anode is eaten away the rest of the exchanger is left intact
What is a disadvantage longer tubes?
Although they are longer so they have a greater surface area, there will be a larger pressure drop through the long tubes
What is a common material for tubes
Copper because of its high thermal conductivity
How are the tubes connected to the tube sheet?
Tube expanding. Sometimes the expanded joints are welded for high pressure or extreme temperature applications.
What it is a tube bundle
The combination of tube sheets and tubes package together
What is the benefits of the U-Tube exchanger
The two bundle can be completely removed for repair or cleaning. However the radius at the end leads to increased wear on the bend
What are the two square type in what are the two triangular type of tubesheet layouts
The square type are 90° or 45° which is just a rotated 90, and the triangular layouts for 30° and 60°. The 30 and 60 allow for more tubes. As there is less room between them, but the 45 and 90 are better for cleaning
What is the purpose of a baffle
It’s two functions are to support the tubes, and to cause turbulence to increase the heat transfer rate and prevent dead bypass areas
What are the two types of baffles?
Segmental baffles, and disc and doughnut baffles
What is an impingement baffle
It is a baffle that sits perpendicular to the shell side inlet. It is to protect the tubes from erosion by the entering high-pressure fluid
What Isa longitudinal baffle
It is used to split the shell side flow into two or more passages
If you have three longitudinal baffles, how many passes are in the shell?
Four. There is always one more pass then baffles
How are the number of tubeside passes determined?
They’re determined by the number of 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 she 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 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 a U-Tube type shell and tube exchanger
Both ends of the U shaped tube are affixed to a single tubesheet. Because of this thermal expansion is not a problem however you can get wear and erosion on the ends of the bend
What is the big advantage of the u-tube exchanger
The entire bundle can be removed for cleaning. There is only one tubesheet joint. Good for high pressure, high temperature. The dirty fluids should run in the shell, not the tube to minimize erosion on the bends
What is an advantage in the disadvantage of the floating head exchanger
And advantage is the pole through floating head is easier maintenance the tube bundle can be removed for shell side cleaning. Disadvantage is there is a large clearances re-quired between the outermost tubes and the inside of the shell this decreases the number of tubes you can put in the shell
What is the floating head with split backing ring?
It has a split backing ring which can be separated therefore reducing the large clearances required. This allows more tubes to be in the shell therefore more efficient
What is the order of disassembly of a split backing ring floating head exchanger
1- shell head
2- floating head cover
3- split ring
4- 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 critical 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 shell?
- dirtier fluid normally flows through the tubes
- The higher pressure fluid normally flows through the tubes
- corrosive fluids should go through the tube
- The cooler fluid usually flows through the shell
What group specifies and standardizes exchanger sizes and parts?
TEMA. Tubular exchanger manufacturers association
What are two shell types?
Split flow, two pass shell with longitude baffle
What efficiency to heat exchangers run at?
If they run at 70% that is very good
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 do you size a tube and shell exchanger
shell diameter/tube length
24” to 197”
Is 24” diameter shell, 197” tube length
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
Why 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 a corrosive fluid should have it flow though which?
The tubes
