Heat and Mass Transfer - Dairy and Food Engineering - Agricultural Engineering Previous Year Questions

1

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A heater is placed in front of a continuous countercurrent dryer. Air at 40 °C and 70% RH is fed into the heater from which the air exits at 65 °C. If saturation vapour pressure at 40 °C and 65 °C are 0.074 bar and 0.250 bar respectively, then relative humidity of the air coming out of the heater and entering the dryer is

A

21%

B

27%

C

32%

D

38%

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2

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A vegetable oil is flowing through a vertical wall as a film. The density and viscosity of the oil are \(920 \text{ kg m}^{-3}\) and \(0.28 \text{ Pa s}\) respectively. If the average velocity of the film is \(0.05 \text{ m s}^{-1}\), the thickness of the film is

A

0.14 mm

B

0.36 mm

C

1.76 mm

D

2.16 mm

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3

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

Air at 101.325 kPa pressure is used to dry a vegetable material at 52 °C. Saturation pressure of water at 52 °C is 13.51 kPa. If the mass transfer coefficient for the case of equimolar counter diffusion (\(k_y\)) is \(4.79 \times 10^{-4} \text{ kg mole m}^{-2} \text{ s}^{-1} \text{ mole fraction}^{-1}\), then, the mass transfer coefficient for the case of diffusion through non diffusing gas (\(k_G\)) in \(\text{kg mole m}^{-2} \text{ s}^{-1} \text{ mole fraction}^{-1}\) is

A

\(4.96 \times 10^{-4}\)

B

\(5.14 \times 10^{-4}\)

C

\(7.83 \times 10^{-4}\)

D

\(1.02 \times 10^{-3}\)

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4

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

Dry bulb and wet bulb temperatures of air fed into a dryer are found to be 60 °C and 35 °C respectively. Saturation humidity at wet bulb temperature is 0.0365 kg H₂O kg dry air⁻¹. If specific heat capacities of dry air and water vapour are 1.008 and 1.915 kJ kg⁻¹ K⁻¹ respectively and latent heat of vaporization at wet bulb temperature is 2.42 MJ kg⁻¹ then humidity ratio of air is

A

0.0193 kg H₂O kg dry air⁻¹

B

0.0225 kg H₂O kg dry air⁻¹

C

0.0256 kg H₂O kg dry air⁻¹

D

0.0275 kg H₂O kg dry air⁻¹

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5

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A refrigerator with a COP of 3.2 uses 2.4 kg min⁻¹ refrigerant extracting 150 kJ kg⁻¹ heat in the evaporator. Assuming compressor efficiency of 85% the minimum size of the motor is

A

0.5 hp

B

1.5 hp

C

2.0 hp

D

3.0 hp

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6

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

If thermal conductivity, mass diffusivity, equimolar mass transfer coefficient based on concentration gradient, density and specific heat capacity of air are 0.03 W m⁻¹ K⁻¹, 2.4 × 10⁻⁵ m² s⁻¹, 0.3 m s⁻¹, 1.0 kg m⁻³ and 1.0 kJ kg⁻¹ K⁻¹ respectively, then convective heat transfer coefficient of air is

A

7.43 W m⁻² K⁻¹

B

74.27 W m⁻² K⁻¹

C

348.12 W m⁻² K⁻¹

D

794.39 W m⁻² K⁻¹

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7

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

Effectiveness of countercurrent heat exchanger is given by
\[ \varepsilon = \frac{1 - \exp\left[-NTU\left(1 - \frac{C_{\min}}{C_{\max}}\right)\right]}{1 - \frac{C_{\min}}{C_{\max}} \exp\left[-NTU\left(1 - \frac{C_{\min}}{C_{\max}}\right)\right]} \]
If same liquid at the same flow rate is used as heating and cooling media through a countercurrent double tube heat exchanger then effectiveness is given by.

A

\(\frac{NTU - 1}{NTU}\)

B

\(\frac{NTU}{NTU + 1}\)

C

\(\frac{NTU - 1}{NTU + 1}\)

D

\(\frac{NTU - 1}{NTU + 2}\)

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8

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

Assuming psychrometric ratio to be unity, milk of 50% total solids is spray dried to powder of 5% moisture content on dry basis. Dry bulb and wet bulb temperatures of the inlet air to the spray dryer are 200 °C and 50 °C respectively. Latent heat of vaporization at the wet bulb temperature is 2393 kJ kg⁻¹. Assuming no sensible heating of powder the outlet air temperature is 80 °C. If inlet air absolute humidity was 0.015 kg H₂O kg dry air⁻¹, then kg of dry air required per kg feed is

A

4.7

B

5.9

C

7.4

D

9.5

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9

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A rectangular fin of length 12 cm, width 22 cm and thickness 1.5 cm is connected to a tube at a temperature of 0 °C. The thermal conductivity of the fin material is 150 W m⁻¹ K⁻¹. The tip of the fin is not insulated. Air at a temperature of 5 °C is in contact with the fin. The heat transfer coefficient between the fin and the air is 25 W m⁻² K⁻¹. The rate of heat transfer is

A

3.33 W

B

6.63 W

C

9.13 W

D

15.23 W

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10

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

In order to reduce heat loss, a steam line with a tube diameter of 1.0 cm is insulated with a material having thermal conductivity of 0.108 W m⁻¹ K⁻¹. Heat is dissipated from the outer surface of the insulating material by natural convection with a heat transfer coefficient of 12 W m⁻² K⁻¹ into the ambient at a constant temperature. The heat loss becomes maximum when the thickness of insulation is

A

0.5 mm

B

2 mm

C

4 mm

D

6.5 mm

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11

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A long cylindrical piece of meat having a diameter of 0.02 m containing 80% moisture is being frozen with air at −30 °C. Initial temperature of the meat is −2.5 °C (freezing point). The heat transfer coefficient of the freezer unit is 20 W m⁻² K⁻¹. If density of the unfrozen meat is 1050 kg m⁻³ and the thermal conductivity of the frozen meat is 1.025 W m⁻¹ K⁻¹, the latent heat of fusion for water is 335 kJ kg⁻¹, shape factors P and R are (1/4) and (1/16) respectively, the freezing time is

A

0.158 h

B

0.373 h

C

0.464 h

D

2.12 h

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12

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

A single effect evaporator is used to concentrate 5000 kg h⁻¹ of a 1.5 wt% sugar solution entering at 50 °C to a concentration of 2 wt% at 101.325 kPa. Steam supplied is saturated at 169.06 kPa (115 °C). The overall heat transfer coefficient is 1550 W m⁻² K⁻¹. The boiling point of solution is the same as that of water. The specific heat of the feed is 4.21 kJ kg⁻¹ K⁻¹. The latent heat of water at 100 °C is 2257.06 kJ kg⁻¹ and the latent heat of steam at 115 °C is 2216.52 kJ kg⁻¹. The required surface area for heat transfer is

A

6.9 m²

B

10.7 m²

C

13.9 m²

D

46.3 m²

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13

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

In a cold store of 30 m × 15 m × 15 m size, 4000 tonnes of potato having the specific heat of 3.62 kJ kg−1 K−1 and heat of respiration of 20 W m−3 is kept at 30 °C. Potato is required to be cooled to 2 °C in 30 days. Neglecting other sources of heat, the capacity of the refrigeration plant required is

A

6 TR

B

38 TR

C

44 TR

D

83 TR

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14

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

Useful heat gain rate for the collector is

A

558.45 W

B

604.35 W

C

657.01 W

D

711.02 W

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15

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

The activation energy for the destruction of Bacillus stearothermophilus is

A

327.56 MJ kg mole⁻¹

B

298.95 MJ kg mole⁻¹

C

208.35 MJ kg mole⁻¹

D

75.62 MJ kg mole⁻¹

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16

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

The z value of the same organism at a reference temperature of 135 °C is

A

9.73 °C

B

10.20 °C

C

10.95 °C

D

11.15 °C

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17

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

The average heat transfer coefficient of the system is

A

22.06 W m⁻² K⁻¹

B

30.52 W m⁻² K⁻¹

C

49.56 W m⁻² K⁻¹

D

53.18 W m⁻² K⁻¹

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18

2007 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2007

The rate of heat transfer at the four surfaces is

A

47.8 kW

B

86.1 kW

C

95.7 kW

D

114.7 kW

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19

2008 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2008

The thermal conductivity of a common metal used in fabrication of food processing equipment is given as 120 BTU ft⁻¹ h⁻¹ °F⁻¹. This value in J m⁻¹ s⁻¹ K⁻¹ will be

A

2.08

B

20.8

C

208

D

280

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20

2008 · Agricultural Engineering · Dairy and Food Engineering · Heat and Mass Transfer

Agricultural Engineering (AG) 2008

Milk and rapeseed oil are flowing in pipes of 5 cm diameter with the same flow velocity of 3 m s⁻¹. The densities of milk and rapeseed oil are 1030 and 900 kg m⁻³, respectively. The viscosity of milk is 2.1 x 10⁻³ N s m⁻² and that of rapeseed oil is 118 x 10⁻³ N s m⁻². The values of Reynolds' number for milk and rapeseed oil will be respectively

A

73571 and 1144

B

1144 and 73571

C

73175 and 1144

D

1414 and 73571

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