A steam pipe is to be insulated by two insulating materials put over each other. For best results
Better insulation should be put over pipe and better one over it
Inferior insulation should be put over pipe and better one over it
Both may be put in any order
Whether to put inferior OIL over pipe or the better one would depend on steam temperature
Correct Answer :
A. Better insulation should be put over pipe and better one over it
Related Questions
Depending on the radiating properties, a body will be opaque when
P = 0, x = 0 and a = 1
P=1, x = 0 and a = 0
P = 0, x = 1 and a = 0
X = 0, a + p = 1 Where a = absorptivity, p = reflectivity, X = transmissivity.
Depending on the radiating properties, body will be transparent when
P = 0, x = 0 and a = 1
P=1, x = 0, and a = 0
P = 0, T= 1, and a = 0
X = 0, a + p = 1
Thermal diffusivity of a substance is
Directly proportional to the thermal conductivity
Inversely proportional to density of substance
Inversely proportional to specific heat
All of the above
A non-dimensional number generally associated with natural convection heat transfer is
Grashoff number
Nusselt number
Weber number
Prandtl number
Heat flows from one body to other when they have
Different heat contents
Different specific heat
Different atomic structure
Different temperatures
In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in
Parallel flow
Counter flow
Cross flow
All of these
According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a
Grey body
Brilliant white polished body
Red hot body
Black body
A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100° C and leaves at 60° C. A cold fluid enters the heat exchanger at 40° C. The mean temperature difference between the two fluids is
20°C
40°C
60°C
66.7°C
Emissivity of a white polished body in comparison to a black body is
Higher
Lower
Same
Depends upon the shape of body
Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by
Convection
Radiation
Forced convection
Free convection
Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of
0.002
0.02
0.01
0.1
Thermal conductivity of water at 20°C is of the order of
0.1
0.23
0.42
0.51
Which of the following statement is correct?
A grey body is one which absorbs all radiations incident on it.
At thermal equilibrium, the emissivity and absorptivity are same.
The energy absorbed by a body to the total energy falling on it, is called emissivity.
A perfect body is one which is black in colour.
Upto the critical radius of insulation,
Added insulation will increase heat loss
Added insulation will decrease heat loss
Convective heat loss will be less than conductive heat loss
Heat flux will decrease
Pick up the wrong case. Heat flowing from one side to other depends directly on
Face area
Time
Thickness
Temperature difference
Which of the following has least value of conductivity?
Glass
Water
Plastic
Air
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
The ratio of surface convection resistance to the internal conduction resistance is known as
Grashoff number
Biot number
Stanton number
Prandtl number
The ratio of the energy absorbed by the body to total energy falling on it is called
Absorptive power
Emissive power
Absorptivity
Emissivity
The most commonly used method for the design of duct size is the
Velocity reduction method
Equal friction method
Static regains method
Dual or double method
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
The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body
Move actually
Do not move actually
Affect the intervening medium
Does not affect the intervening medium
The heat transfer by conduction through a thick sphere is given by
Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)
Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)
Heat transfer by radiation mainly depends upon
Its temperature
Nature of the body
Kind and extent of its surface
All of the above
Kirchhoff's law states that
The total radiation from a black body per second per unit area is directly proportional to the fourth power of the absolute temperature
The wave length corresponding to the maximum energy is proportional to the absolute temperature
The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body
None of the above
Heat is closely related with
Liquids
Energy
Temperature
Entropy
Which of the following would lead to a reduction in thermal resistance?
In conduction, reduction in the thickness of the material and an increase in thermal conductivity.
In convection, stirring of the fluid and cleaning the heating surface.
In radiation, increasing the temperature and reducing the emissivity.
All of the above
A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be
k₁ k₂
(k₁ + k₂)
(k₁ + k₂)/ k₁ k₂
2 k₁ k₂/ (k₁ + k₂)
According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to
Absolute temperature
Square of temperature
Fourth power of absolute temperature
Fourth power of temperature
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