Heat Exchangers Flashcards
why are finned tube heat exchangers not commonly used for intercooling on large compressor installations?
they would have to be extremely large to deal with the high pressures and temperatures
what is conduction?
the transfer of head through contact of the fluid and a metal surface
what does it mean to have forced convection in a heat exchanger?
using a fan or pump to move the heat
which type of flow is most desirable in a heat exchanger laminar or turbulent?
turbulent
which type of flow path is more efficient, counter flow, cross flow, or parallel?
counter flow
why is copper commonly used in heat exchangers?
very high thermal conductivity
what type of exchanger is the common radiator?
finned tube exchanger
why are the plates placed close together on the plate heat exchangers?
created a high heat transfer because of the relative large surface area
why is the spiral heat exchanger so efficient?
because the fluids have a true counterflow path
why is part of the water evaporated in a cooling tower?
because of the temperature change
an ___________ cooling tower has a fan that forces air OUT of the tower
induced
the heat exchanger that has a single pipe placed concentrically inside a larger pipe is called the ________________ exchanger
double pipe
what is the purpose of a zinc anode in a heat exchanger?
he’s a sacrifice, so the electrolyte action of the two fluids eats away at the zinc and not the pipes
what is the main disadvantage of using longer tubes in a shell and tube exchanger compared to short tubes
pressure drop
is the triangular or the square pitch tube pattern more efficient for heat transfer in a shell and tube exchanger?
triangular
baffles in shell and tube heat exchangers commonly serve two functions, what are they?
support the tubes
create turbulence
prevent dead by-pass areas
why are baffles necessary for heat transfer in large shell and tube exchangers?
- to support the tubes from vibration
- to prevent fluid from taking the path of least resistance
- surface area to cool the liquid
which baffles are normally provided over the tubes to reduce erosion by the entering fluid?
impingement baffles
which baffles are used to split the shell side flow into two or more passes, allowing more contact time between fluids?
longitudinal baffles
what determines the number of tube side passes?
the number of baffles in the channel head
which type of exchanger is commonly used in services with a low differential of thermal expansion?
fixed tubesheet
which type of exchanger has the main disadvantage that the shell side cannot be mechanically cleaned?
fixed tube exchanger
tube heat exchangers should have the dirty fluids routed on the __________ side since the bundle cannot be removed for cleaning. the ___________ side is restricted to clean fluids.
shell
tube
what is the main advantage of the floating heat exchanger compared to the fixed tube type of exchanger?
allow for thermal expansion
what is the major disadvantage of the floating heat exchanger compared to the floating head with a baking device type of exchanger?
large clearance at the head end of the pull through type
which applications is it desirable to use a double tubesheet exchanger?
when the two fluids cannot mix
why do smaller diameter tubes result in a more efficient heat transfer rate?
they have increased velocity which increases the turbulence
an increase in pressure drop through the exchanger or a drop in heat exchange efficiency usually indicates __________________
cleaning is needed
why should the flange bolts always be loosened at the bottom of the flange first when performing heat exchanger maintenance?
so the heat doesn’t bow into your face, it goes to the ground
impingement baffle is located below the ___________ linlet line and it prevents erosion of the _____________ of the tubes
shell
outside
zinc anodes are used to control _____________ action
electrolytic
longitudinal baffles are used to increase _______________ pass
shell side
what are drift eliminators in cooling towers used for?
capture most of the water that is being pulled out with airflow
fins transfer heat from the tube by _______________
conduction
list three purposes for segmental baffles
support the tubes
cause turbulence
prevent dead by-pass areas
