HeatX Flashcards
What flow path within a heat exchanger will reach an equilibrium in temperature half way between the two extremes
Parallel flow
What flow path within a heat exchanger will reach the cooler of the two temperature extremes given enough time
Counter flow
What flow path has the greatest efficiency for a given size
Counter flow
What is the primary purpose of the fins on a finned tube exchanger
To increase surface areas
Which of the heat exchangers has very efficient true counter flow
Spiral because of its continuous curving passages it creates high turbulence.
In turn the velocity of the fluid scrubs it clean. This exchanger is an excellent choice for fluids that tend to deposit sludge
Within cooling towers the heat exchange is dependent on
Evaporation
What is the purpose of the zink anodes in a shell and tube heat exchanger
To minimize electrolytic action within the heat exchanger
What heat exchanger has its capacity changed the easiest.
Plate
Cooling towers are commonly used to do what
Recycle water, the water to be cooled is pumped to the top of the tower where it is sprayed through a Nozel to give a fine spray, some evaporates cooling the remainder
What baffle is used to reduce erosion by entering fluids
Impigmant
What are the two common functions of shell side baffles
To increase turbulent flow and support the tube bundle
How many longitudinal shell baffles are required to get four passes
3
Which heat exchanger is limited to chemical cleaning of the tube side
U-tube
What heat exchanger is most likely to suffer from thermal stress causing tube sheet leakage
Fixed tube sheet
What’s the main advantage and disadvantage of the floating head exchanger
The advantage is it’s easier to clean
The disadvantage is the large clearance requirements needed
What is a disadvantage to the floating head outside-packed, lantern ring exchanger
If the exchanger leaks the fluids are exposed to the atmosphere
Where is a double tube sheet exchanger used
In applications where is is not desirable to have mixing of tube-side and shell-side fluids
It is impossible for this set up to have any mixing and will drip externally
What fluid usually flows through the tube side
The dirtier fluid as it’s easier to clean through the tubes
Higher pressure fluid flows through tubes of shell side?
Tube side as it could possible collapse the tubes if it flowed through shell side
The fluid with the lower heat transfer usually travels where?
The outside of the tubes, it’s cheaper and easier to add fins for heat transfer
What is TEMA
Tubular Exchanger Manufacturers Assosiation
What is the advantage of a floating head with split backing device
Allows for more tubes to fit in the shell while allowing for thermal growth
Heat transfer is primarily effected by what 5 causes
Fluid flow Temperature differentials Hearing surface area Contact time Conductivity
Other factors would consist of Fluid films
Thickness of heating surface
Cleanliness of hearing surface
What is a surface layer
A film or a fluid stuck to the metal wall
Cold start up is when….
You gradually start both fluids at the same time
Shutdown is when you….
Gradually stop both fluids at the same time
What are the three most common causes to Poor performance in heat exchangers
Leaking tube sheets
Fouling
Vibration
An increase in resistance to fluid flow and heat transfer due to undesired deposits is referred to as
Fouling
Deposit build up lowers heat transfer rate
What should you never use to clean a heat exchanger
Steam
It’s ok to screw eye bold I to the tube sheets
Yes, tubes are thin and bend easy so rigging on each end is most desirable
Stress cracks in tubes are a sign of what
High stresses caused by continuous vibrations
What is the purpose of hydrostatic testing
Used to check heat exchangers for leaks before and after overhauling - the standard for hydrostatic testing is 1-1/2 times the working pressure
What precaution should be taken when loosening head studs.
Ensure exchanger has been purged vented and drained
Loosen bolts at the bottom first
What happends if you check leaks with hot water
Expansion could seal the leak
What pressure is normally used for hydrostatic pressure
1-1/2 times the working pressure
How is the shell side of an exchanger cleaned.
Hot water or chemical
What is the advantage of a pull through floating head exchanger
Larger clearance around tube bundle = less surface area
How could you tell if there was a tube sheet leak on a double tube sheet exchanger
Drips between the two tube sheets
She’ll and tube exchanger sizes are classified by
Shell Dia. and tube length
Define the purpose of a heat exchanger.
To cool or heat liquid through a variety of processes
Define conduction
The transfer of heat energy from one molecule to another or contact of fluid and a metal
Define Convection
The result of the movement of mass of fluid (gas or liquid)from a warm location to a cooler one
Define radiation
The transfer of energy by electro magnetic wave
Different materials have different rates of thermal expansion
What potential problems in a heat exchanger are related to this.
Loosening of joints
Different materials have different rates of thermal expansion
Name two exchanger designs that illuminate this problem
Utube
Floating head
Three advantages of uTube exchanger
Both ends are fixed illuminating thermal expansion problems
Easy to remove and clean
Well suited for high-temp/high pressure
What’s the most common way a tube is attached to a tube sheet
Expansion into tube sheet
Rolled in
And welding are the other two types
The rate of heat transfer is effected by
Temperature difference
Area of contact
Conductivity of metal
Nature of substance
What flow is most desirable in a heat exchanger
Turbulent flow
Scrubs the walls allowing more heat transfer
Mixed flow is common where
In heat exchangers with baffles that cause the flows to Switch between parallel and counter flow
What flow PATH is more desirable
Counter flow is more efficient
What type of exchanger is a common radiator
Finned tube
Greater surface area
Why are spiral heat exchangers so efficient
Both fluids have true contour
Dead bypasses are illuminated
Less fouling
Why is part of the water evaporated in the cooling tower
The heat to evaporate is drawn from the remaining water
Resulting in a temp drop in the reaming water
What type of cooling tower has a fan that forces the water out
Induced draft
The heat exchanger that has a pipe concentric inside another pipe is called
Double pipe
Perfect for high pressure
Designed for counter flow current
What baffles are normally provided over the tubes to reduce erosion by the entering fluid
Impigman baffles
What baffle is used to split the shell side flow
Longitudinal
Mixed flow is common where
In heat exchangers with baffles that cause the flows to Switch between parallel and counter flow
What flow PATH is more desirable
Counter flow is more efficient
What type of exchanger is a common radiator
Finned tube
Greater surface area
Why are spiral heat exchangers so efficient
Both fluids have true contour
Dead bypasses are illuminated
Less fouling
Why is part of the water evaporated in the cooling tower
The heat to evaporate is drawn from the remaining water
Resulting in a temp drop in the reaming water
What type of cooling tower has a fan that forces the water out
Induced draft
The heat exchanger that has a pipe concentric inside another pipe is called
Double pipe
Perfect for high pressure
Designed for counter flow current
What baffles are normally provided over the tubes to reduce erosion by the entering fluid
Impigman baffles
What baffle is used to split the shell side flow
Longitudinal
What is the main disadvantage of the floating head exchanger compared to the floating head with the baking device
The disadvantage of the pull through exchanger is the the large clearance requirements between outer tubes and shell
