HMT Flashcards
The driving force in heat transfer is
temperature gradient
Under the same temperature gradient across various metal plates of the same thickness, the heat conducted per unit of surface area will be largest across
copper
If a man touches two metals which were kept together at room temperature, why would one metal feel colder than the other
one has a high thermal conductivity
Cork is a good insulator because it has
porous body
The insulation ability of an insulator with the presence of moisture would ___
Decrease
Heat transfer in turbulent flow may be described by an empirical equation correlating
Nusselt, Prandtl, and Reynold’s number
When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by ____
radiation
An ideal surface that absorbs all incident radiation, regardless of the wavelengths and direction and is also considered to be a perfect emitter is referred to as a ____
black body
Which type of body is independent of temperature and wavelength?
graybody
In thermal radiation, the absorptivity and emissivity of a black body is ___
α= 1 and ε= 1
The Fick’s Law of Diffusion gives the rate of diffusion based on ____
a. pressure driving force
b. temperature driving force
c. concentration driving force
d. all of these
concentration driving force
It provides the basis for the quantitative description of the diffusional phenomena of molecules
Kinetic Theory of Gases
The enrichment of ethyl alcohol in the vapor phase from an aqueous ethyl alcohol solution during rectification is an example of ____
equimolal counter-diffusion
It is a measure of the diffusive mobility of individual molecules in solution
diffusion coefficient
Ammonia (A) being absorbed from air (B) into water is an example of ____
steady state unicomponent diffusion
For a gas phase diffusion, unicomponent diffusion through a gas-liquid interface will likely to occur if ____
one component is soluble and the other is not
For binary unicomponent diffusion, the concentration profile of the diffusing component along the diffusion path is ____
logarithmic
Molecular diffusivity of a liquid ____ with temperature
Increases with temperature
The method of solute transfer which involves rapid movement of large chunks of fluid in turbulent motion
eddy diffusion
The flux of A in unicomponent diffusion is different from that of equimolal counter-diffusion because of
the bulk motion of the phase
It represents the total mass transferred to mass transferred by molecular diffusion.
Sherwood Number
For turbulent mass transfer in pipes, the Sherwood number depends upon the Reynolds number (Re) as
Re^0.83
A dimensionless number that represents the ratio of the molecular momentum diffusion to the molecular mass diffusivity
Schmidt number
Unit operation that involved the concentration of a solution consisting of a non-volatile solute and a moisture solvent.
Evaporation
Evaporation is also sometimes called as
water distillation
The concentration of glycerin from a water solution by the application of heat is _____
Evaporation
The main function of baffles provided in a shell and tube heat exchanger is to
Enhance turbulence
The total amount of evaporation an evaporator is capable of producing per unit of time
Capacity
Evaporator used when the feed is hot and temperature sensitive.
Forward feed
Evaporator where pumping is not required
Forward feed
Evaporator used when feed is cold, and saving a large quantity of steam and increased economy is desired.
Backward feed
Evaporator when the upsides of backward and forward feed it desired
Mixed feed
Evaporator where feed is introduced in middle effect. It moves forward to next effect and backward to 1st effect
Mixed feed
Evaporator where fresh feed is introduced in every effect
Parallel feed
In evaporation, it is a type of feeding where the fresh solution introduced in the last effect and goes to the preceding effect where its concentration is highest.
Backward feeding
For a cold viscous feed, backward feed gives __________ than forward feed.
A. A higher capacity
B. A lower capacity
C. Lower economy
D. None of these
A. A higher capacity
The suitable evaporator for concentration of foaming liquids is a __________ .
Long tube evaporator
“Boiling point of a given solution is a linear function of the boiling point of pure water at the same pressure”
Duhring’s Rule
Boiling point rise is independent of ___
operating pressure
When the pressure of the chamber is decreased, the boiling point of the solution inside the chamber will ____
decrease
A heat transfer device that is normally used to liquefy vapors is ___
condenser
The temperature driving force in an evaporator is determined as the difference between the condensing steam temperature and
boiling point of the solution
In a country where capital is scarce and energy cost is low, which is appropriate to install?
single effect evaporator
Compare capital cost and energy cost of single effect and multiple effect evaporators
Single effect: low capital, high energy (no vapor recycling)
Multiple effect: high capital (marami haha), low energy
The fresh feed enters at the coldest effect.
feed-backward
In commercial practice, the areas in all effects for a multiple effect evaporator are ____
Constant
A multiple effect evaporator ______the steam cost
decreases
___ of the vapor is recovered and reuse in the multiple effect evaporator.
latent heat
It is the deposition and growth on body walls, and specially on heating surfaces, of a material undergoing an irreversible chemical reaction in the evaporator.
scaling
It is the formation of deposits other than salt or scale and maybe due to corrosion.
Fouling
The unit operation for separating a particular gaseous component from a mixture of gases due to difference in solubility of the gases in a liquid phase
Absorption
In a gas absorption tower, mass transfer occurs primarily from ___ to ____
Vapor phase to Liquid phase
Separation of a mixture of two gases by absorption in the liquid solvent depends upon the difference in their ___
solubility
In gas absorption, one solute is being absorbed while the rest of the components are assumed to be ______ and the liquid is ________.
components are inert
liquid is non-volatile
Poor performance in large packed towers is due mainly to ____
Channeling (falling liquid will follow channels where some portion of the pack is dry, less contact with the liquid)
Ideal Operating Pressure and Temperature:
Gas Absorption
Low T, High P
(V->L: Desires high gas solubility)
In general, operating pressure should be ___ and temperature ___for an absorber, to minimize stage requirements and/or absorbent flow rate to lower the equipment volume required to accommodate the gas flow.
high P, low T
Ideal Operating Pressure and Temperature:
Stripping / Desorption
High T, Low P
(L->V: Desired low gas solubility)
What could be the effect of lowering the temperature of stripping?
Increased number of stages
The reverse is true for stripping. However, the operating pressure should not be too high and the operating temperature should not be too low as to ___ the feed gas.
condense the feed gas
Flooding results in ___
low tray efficiency
Operating velocity in a packed tower is usually ___
half the flooding velocity
Flooding in a column results due to ____
Channeling in a packed tower results from ____
low tray efficiency
Process whereby a solution becomes supersaturated so as to cause the formation of crystals
Crystallization
In industrial crystallization from a solution, the two phase mixture of mother liquor and crystals of all sizes, which occupies the crystallizer and is withdrawn as product is called ___
magma
In practice, slow cooling maintaining a low level of supersaturation produces ___ crystals and fast cooling produces ___ crystals.
large, small
It is the process by which crystalline bodies are created within a supersaturated fluid.
nucleation
When a solution is cooled to produce a supersaturated solution and hence to cause crystallization, the heat that must be removed is the sum of the ____ necessary to cool the solution and the heat of crystallization.
sensible heat content
Crystallization Equipment:
1. by Heating
2. by Cooling
3. by Vacuum
4. by Seeding
- by Heating
Oslo Crystallizer - by Cooling
Swenson-Walker, Pan Coolers, Agitated Batch Coolers - by Vacuum
Vacuum Crystallizers - by Seeding
Mixed-Suspension Mixed-Product Removal (MSMPR)
Which of the following processes may always be regarded as a constant-humidity process?
a. cooling of air
b. air conditioning
c. water-cooling in a cooling tower
d. heating of air
heating of air
(no condensation)
Wet bulb and dry bulb temperatures become identical at _____
100% RH (full saturation)
Steady state temperature reached by a small amount of liquid evaporating into a large amount of unsaturated vapor-gas mixture is _____
Wet-bulb temperature
Explain wet bulb temperature
Steady state temperature reached by a small amount of liquid evaporating into a large amount of air
The degrees of freedom associated with a humidification process is __
2
See Psychrometric Charts
What happens at dew point?
Air gets saturated with water (100% RH%). Water condenses.
Which of the following remains constant during sensible cooling process?
specific humidity & partial pressure of vapor
During sensible cooling process, ____
relative humidity & wet bulb temperature increases
Transfer of liquid from a wet solid into an unsaturated gas phase
Drying
For drying, the moisture content at end of constant rate period and beginning of fall rate period is ___
Critical moisture, Xc
To remove all the moisture from a wet solid requires exposure to ____
a. perfectly dry air
b. air at high temperature
c. highly humid air
d. none of these
Perfectly dry air
Moisture contained by a substance in excess of the equilibrium moisture is called the __________ .
Free moisture
Moisture in a substance exerting an equilibrium vapour pressure less than that of pure liquid at the same temperature is called the __________ .
Bound moisture
Moisture content of a substance when at equilibrium with a given partial pressure of the vapour is called the __________ .
Equilibrium moisture
Moisture in a solid exerting an equilibrium vapour pressure equal to that of the pure liquid at the same temperature is called the __________ .
Unbound moisture
Why, in actuality, we cannot perfectly dry materials?
Because air has moisture content. Perfectly dry air need to perfectly dry materials.
When a wet solid is brought in contact with air, the solid tends to lose moisture if the air is ___
has lower humidity than the humidity corresponding to the moisture content of the solid
The portion of water in the wet solid that cannot be removed by the air in its contact is called _____
equilibrium moisture content
Moisture contained by a substance in excess of the equilibrium moisture is called ____
Free moisture
When drying banana chips under the sun, the rate of drying is faster on a ___
hot and windy day
What happens during constant rate and falling rate drying periods?
CRP: constant rate because surface moisture is readily replenished by moisture underneath the surface
FRP: surface moisture no longer replenished.
The critical moisture content in case of drying indicates ____
beginning of falling rate period
NOT IN ANSWER: or end of constant rate period
In drying at the falling rate period, the rate at which moisture is removed from the solid material ____
decreases with time
Used to cool large amounts to liquids
Cooling tower
Used for drying pasty materials such as wet filter cakes and lumpy solids
tray drier
Used for drying pastes
Drum driers
or Rotary Vacuum Dryer
Heat sensitive materials like certain pharmaceuticals and food stuff can be dried by ____
freeze drying
or vacuum drying
Other term for freeze drying.
Lyophilization
Detergent solution is dried to a powder in a ____
Spray dryer
Calcium ammonium nitrate (a fertilizer) is dried in a _____
rotary dryer
Rotary dryer cannot handle ____
Sticky materials
Describe temperatures in ADIABATIC DRIER
Temperature of the solid is equal to the wet bulb temperature of air
The rate of evaporation at a given operating temperature in an evaporator is more when ____
the operating pressure is low
The mode of heat transfer that can take place in a vacuum is known as ___
Radiation
What do you call the effectiveness of a body as a thermal radiator at a given temperature?
emissivity
Viscosities can change with time assuming all other conditions to be constant. If the viscosities increases with time up to a final value how do you call the fluid?
rheopectic fluid
“The amount of radiation emitted in all directions and overall niblings by a perfect emitter is proportional to the fourth power of the absolute temperature of the emitter.”
Stefan Boltzmann Law
An ideal surface that absorbs all incident radiation regardless of the wavelength and direction and is also considered as a perfect emitter is referred to as ____
black body
