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Solved Examples on Heat Phenomena:-

Question 1:-

(a) Calculate the rate of heat loss through a glass window of area 1.4 m2 and thickness 3.0 mm if the outside temperature is -20º F and the inside temperature is +72º F.

(b) A storm window is installed having the same thickness of glass but with an air gap of 7.5 cm between the two windows. What will be the corresponding rate of heat loss presuming that conduction is the only important heat-loss mechanism?

Concept:-

The rate H at which heat is transferred through the rod is,

(a) directly proportional to the cross-sectional area (A) available.

(b) inversely proportional to the length of the rod Δx.

(c) directly proportional to the temperature difference ΔT.

So, H = kA ΔT/ Δx    

Here k is the proportionality constant and is called thermal conductivity of the rod.

Again the rate of heat transfer H is defined as,

H = A(T2-T1)/∑Rn

    = A ΔT/ (Rg+ Ra)

Here A is the area,  ΔT  is the temperature difference,  Rg is the thermal resistance of glass at the thickness 3.0 mm and Ra is the thermal resistance of air at the thickness 7.5 cm.

The thermal resistance or R-value, defined by,

R = L/k

Here L is the thickness of the material through which the heat is transferred and k is the thermal conductivity of the material.

Solution:-

(a) First we have to find the temperature difference between inside and outside of the glass window.

If T2 is the inside temperature and T1 is the outside temperature, then the temperature difference ΔT in °C will be,

ΔT = T2-T1

     = 5°C /9 °F (72°F –(-20 ° F))

     = 51.1  °C

     = (51.1+273) K

     = 324.1 K

To obtain the rate of heat loss through a glass window, substitute 1.0 W/m.K (thermal conductivity of window glass) for k, 1.4 m2 for A, 324.1 K for ΔT and 3.0 mm for Δx in the equation H = kA ΔT/ Δx, we get,

 H = kA ΔT/ Δx

     = (1.0 W/m.K) (1.4 m2) (324.1 K)/ (3.0 mm) 

     = (1.0 W/m.K) (1.4 m2) (324.1 K)/ (3.0 mm×10-3 m/1 mm)

    = 2.4×104 W

From the above observation we conclude that, the rate of heat loss through a glass window would be 2.4×104 W.

(b) First we have to find out the thermal resistance Rg of galss at the thickness 3.0 mm and Ra of air at the thickness 7.5 cm.

To obtain the thermal resistance Rg of galss at the thickness 3.0 mm, substitute 3.0 mm for L and 1.0 W/m.K (thermal conductivity of window glass) for k in the equation R = L/k,

Rg = L/k

     = 3.0 mm/1.0 W/m.K

    = (3.0 mm×10-3 m/1 mm) /(1.0 W/m.K)

    = 3.0×10-3 m2.K/W

To obtain the thermal resistance Ra of air at the thickness 7.5 cm, substitute 7.5 cm for L and 0.026 W/m.K (thermal conductivity of dry air) for k in the equation R = L/k,

Ra = L/k

     = 7.5 cm /0.026 W/m.K

    = (7.5 cm ×10-2 m/1 mm) /(0.026 W/m.K)

  = 2.88 m2.K/W

To find out the corresponding rate of heat loss H, substitute 1.4 m2 for A,3.0×10-3 m2.K/W for Rg and 2.88 m2.K/W for Ra in the equation H = A ΔT/ (Rg+ Ra), we get,

H = A ΔT/ (Rg+ Ra)

    = (1.4 m2) (324.1 K) /(3.0×10-3 m2.K/W) (2.88 m2.K/W)

    = 25 W

From the above observation we conclude that, the corresponding rate of heat loss H would be 25 W.

______________________________________________________________________________________________

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Question 2:-

What mass of steam at 100ºC must be mixed with 150 g of ice at 0ºC, in a thermally insulated container, to produce liquid water at 50ºC?

Concept:-

The heat capacity per unit mass of a body, called specific heat capacity or usually just specific heat, is characteristic of the material of which the body is composed.

c = C/m

  = Q/mΔT

So, Q = c mΔT

Here, the heat transferred is Q, specific heat capacity is c, mass is m and the temperature difference is ΔT.

The amount of heat per unit mass that must be transferred to produce a phase change is called the heat of transformation or latent heat L for the process. The total heat Q transferred in a phase change is then,

Q = Lm

Here m is the mass of the sample that changes phase.

Solution:-

The heat given off the steam Qs will be equal to,

Qs = msLv+ mscwΔT

Here, mass of steam is ms, latent heat vaporization is Lv, specific heat capacity of water is cw and the temperature difference is ΔT.

The heat taken in by the ice Qi will be equal to,

Qi = miLf+ micwΔT

Here, mass of ice is mi, latent heat fusion is Lf, specific heat capacity of water is cw and the temperature difference is ΔT.

Heat given off the steam Qs is equal to the heat taken in by the ice Qi.

So, Qs = Qi

msLv+ mscwΔT = miLf+ micwΔT

ms(Lv+ cwΔT) = mi(Lf+ cwΔT)

ms = mi(Lf+ cwΔT)/ (Lv+ cwΔT)

To obtain the mass of the steam at 100 °C must be mixed with 150 g of ice at 0 °C, substitute 150 g for mass of ice mi, 333×103 J/kg for Lf, 4190 J/kg.K for cw, 50° C for ΔT, 2256×103 J/kg for Lv in the equation ms = mi(Lf+ cwΔT)/ (Lv+ cwΔT), we get,

ms = mi(Lf+ cwΔT)/ (Lv+ cwΔT)

=(150 g)[(333×103 J/kg) +(4190 J/kg.K) (50° C)]/ [(2256×103

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