Moisture would find its way into insulation by vapour pressure unless it is prevented by
High thickness of insulation
High vapour pressure
Less thermal conductivity insulator
A vapour seal
Correct Answer :
D. A vapour seal
Related Questions
Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called
Thermal resistance
Thermal coefficient
Temperature gradient
Thermal conductivity
Thermal diffusivity of a substance is given by (where h = Thermal diffusivity, ρ = Density of substance, S = Specific heat, and k = Thermal conductivity)
h = k/ ρS
h = ρS/k
h = S/ρk
h = kρ/S
The unit of overall coefficient of heat transfer is
W/m²K
W/m²
W/mK
W/m
Conduction is a process of heat transfer
From one particle of the body to another without the actual motion of the particles
From one particle of the body to another by the actual motion of the heated particles
From a hot body to a cold body, in a straight line, without affecting the intervening medium
None of the above
Thermal conductivity of glass wool varies from sample to sample because of variation in
Composition
Density
Porosity
All of the above
The ratio of the energy absorbed by the body to total energy falling on it is called
Absorptive power
Emissive power
Absorptivity
Emissivity
The unit of overall coefficient of heat transfer is
kcal/m²
kcal/hr °C
kcal/m² hr °C
kcal/m hr °C
A grey body is one whose absorptivity
Varies with temperature
Varies with the wave length of incident ray
Varies with both
Does not vary with temperature and wave length of the incident ray
The expression Q = ρ AT4 is called
Fourier equation
Stefan-Boltzmann equation
Newton Reichmann equation
Joseph-Stefan equation
Thermal conductivity of non-metallic amorphous solids with decrease in temperature
Increases
Decreases
Remain constant
May increase or decrease depending on temperature
LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is
Higher
Lower
Same
Depends on the area of heat exchanger
A grey body is one whose absorptivity
Varies with temperature
Varies with wavelength of the incident ray
Is equal to its emissivity
Does not vary with temperature and. wavelength of the incident ray
The transfer of heat by molecular collision is smallest in
Solids
Liquids
Gases
None of these
Thermal conductivity of wood depends on
Moisture
Density
Temperature
All of the above
The amount of heat flow through a body by conduction is
Directly proportional to the surface area of the body
Directly proportional to the temperature difference on the two faces of the body
Dependent upon the material of the body
All of the above
Unit of thermal conductivity in S.I. units is
J/m² sec
J/m °K sec
W/m °K
Option (B) and (C) above
The energy distribution of an ideal reflector at higher temperatures is largely in the range of
Shorter wavelength
Longer wavelength
Remain same at all wavelengths
Wavelength has nothing to do with it
Fourier's law of heat conduction is (where Q = Amount of heat flow through the body in unit time, A = Surface area of heat flow, taken at right angles to the direction of heat flow, dT = Temperature difference on the two faces of the body, dx = Thickness of the body, through which the heat flows, taken along the direction of heat flow, and k = Thermal conductivity of the body)
k. A. (dT/dx)
k. A. (dx/dT)
k. (dT/dx)
k. (dx/dT)
Sensible heat is the heat required to
Change vapour into liquid
Change liquid into vapour
Increase the temperature of a liquid of vapour
Convert water into steam and superheat it
In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by
Hr (time)
Sq. m (area)
°C (temperature)
K.cal (heat)
In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.
Same
Less
Greater
None of these
The heat of sun reaches to us according to
Conduction
Convection
Radiation
None of these
The total emissivity power is .defined as the total amount of radiation emitted by a black body per unit
Temperature
Thickness
Area
Time
The rate of energy transferred by convection to that by conduction is called
Stanton number
Nusselt number
Biot number
Peclet number
According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the
Absolute temperature
Square of the absolute temperature
Cube of the absolute temperature
Fourth power of the absolute temperature
Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.
Cold body to hot body
Hot body to cold body
Smaller body to larger body
Larger body to smaller body
According of Kirchhoff's law
Radiant heat is proportional to fourth power of absolute temperature
Emissive power depends on temperature
Emissive power and absorptivity are constant for all bodies
Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.
In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by
Convection
Radiation
Conduction
Both convection and conduction
Thermal conductivity of a material may be defined as the
Quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C
Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C
Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces
All of the above
Heat is closely related with
Liquids
Energy
Temperature
Entropy