FINAL Flashcards
What are the two things that the cooling rate depends on?
film resistance at surface (h) and rate of heat flow out of interior
What is ks?
thermal conductivity
What is Ts?
T at any point of object
What is Fourier number?
dimensionless measure for conduction
What does Fourier number account for?
cooling time and size of object
What is Biot number measure of?
relative importance of surface and interior resistance terms
Bi =
interior resistance/surface resistance
Describe resistance of small Bi
all resistance in film
Describe resistance of intermediate Bi
both resistances important
Describe resistance of large Bi
all resistance in conduction through solid
What is lumped parameter analysis?
analysis of system that has uniform properties throughout
What Bi is considered small?
Bi < 0.1
What Bi is considered large?
Bi > 40
What happens when negligible surface resistance?
surface immediately drops to T of fluid and conduction within object important
The approach to equilibrium (final T) takes a longer time for….
larger surface resistance (smaller Bi number)
Short cooling times mean..
heat is only lost in outer layer of solid and cooling has not yet reached deep in object
What is true of h (in Bi number) when heat enters/leaves body by both convection and radiation?
should be overall coefficient accounting for both mechanisms of heat transfer
h(overall) = h(convection) + h(radiation)
What will happen to the radiation coefficient as the surface temperature of a particle changes?
it will change considerably
Heat exchangers are devices for…
transferring heat from a hot flowing stream to a cold flowing stream
What are the three broad types of exchangers?
recuperator (aka through the wall non-storing), direct-contact, regenerator (aka accumulator or heat-storing)
How does one decide on what type of exchanger to use?
depends on nature of transferring phases and mutual solubility of phases involved
Describe the streams and movement of heat of recuperator
streams separated by wall and heat has to pass through wall
How does recuperators effectiveness compare to direct-contact exchanger?
less effective because presence of wall hinders flow of heat
When is it smart to use recuperator?
when fluids not allowed to contact each other, gas-gas systems, miscible liquids, dissolving liquids, or reactive chemicals
Describe the streams and movement of heat of direct-contact non-storing exchangers
streams contact each other and the hotter stream gives up heat directly to colder stream
When is it smart to use direct-contact non-storing exchanger?
when 2 contacting phases mutually insoluble and do not react with each other (cannot be used for gas-gas systems)
What are the three types of direct-contact exchangers?
gas-solid, fluid-fluid, air-water
Describe the fluids of fluid-fluid direct-contact exchangers
mutually immiscible
What is the outlier of direct-contact exchangers? Why?
air-water systems because the water evaporates in the air
Describe the streams and movement of heat of direct-contact heat storing exchangers
hot stream of gas transfers heat to intermediary (usually solid) that later gives up stored heat to second stream of cold gas
What are the two important factors to understand when dealing with recuperators?
overall heat transfer coefficient (U), contacting pattern of the two phases
What does U account for in recuperator?
overall resistance to transfer caused by wall (included individual film resistances and wall resistance)
For shell and tube exchangers, M C and T refer to…
shell-side
For shell and tube exchangers, m c and t refer to…
tube-side
How is the amount of heat exchanged as the fluids pass through the exchanger measured?
enthalpy change of one of the fluids or as fraction of total heat transferred
How to differentiate efficiency equation of countercurrent and co-current exchangers?
cocurrent as K’
What kind of flow is more efficient?
countercurrent flow
In the absence of shaft work, what is the energy balance of a flowing fluid?
DeltaH = Q
Why do we use other shell and tube exchangers besides countercurrent?
others are more convenient, compact, and less expensive
What will the fudge factor be?
between 0 and 1
Why do we use fudge factor in shell and tube exchangers?
to account for lowered contacting efficiency
What is true of the change in P for shell side v tube side? What does this tell us about the fluids?
tube-side has greater change in pressure, so less viscous fluid should pass through tubes
The larger the number of shell and tube passes…
the closer flow is to approaching ordinary countercurrent flow with its largest T driving force
What are the two types of flow for a phase of crossflow and compact exchangers?
well mixed laterally or unmixed
How will a fluid with no lateral mixing act in crossflow and compact exchangers?
hot fluid will take separate and parallel paths through exchanger and hot fluid at B will be much cooler than hot fluid at A
How will a fluid with lateral mixing act in crossflow and compact exchangers?
all fluid along AB will be at one and the same T
Is unmixed or mixed flow better?
unmixed
Is single-pass or multi pass contacting better?
multi-pass (if done properly)
How to find size of exchanger?
noting the terminal temperatures of the exchanger
What is the difference between Nusselt and Biot numbers?
Bi is ratio of internal resistance to surface resistance while Nu is ratio of convective heat transfer to conductive heat transfer
Describe graph of using hot oil bath to heat up solid spherical particle (Bi = 100) w T on y axis and L on x axis
concave up parabola
Describe graph of using hot oil bath to heat up solid spherical particle (Bi = 100) w T on y axis and L on x axis at different times
T increase with time and parabolas could get flatter since particle increasing temp
In a shell and tube heat exchanger, how is the heat flow in the shell related to the heat flow in the tubes? Why?
each Q is equal but opposite in sign and heat flows from one to the other
If the overall heat transfer coefficient for a fluid in heat exchanger increases, how would you expect that to change necessary surface area of interaction between hotter and colder fluids? Why?
if U increases, A will decrease because of Q eqn
