EXERCiSES

7.1 An industrial process heating system uses air heated by both solar and auxiliary energy. It enters the duct supplying the air to the process at 37°C. The solar heat is supplied from a storage tank and transferred to the air via a water-to-air heat exchanger with an effectiveness equal to 0.95. The air temperature is topped by auxiliary energy to 60°C. The collector area is 70 m2, Fr(to>) = 0.82, FRUL= 6.15 W/m2-°C. The fully mixed storage tank has 4.5 m3 capacity and its UA value is 195 W/°C; it is located in a room with temperature of 18°C. The capacitance of the collector side of the heat exchanger is 1150 W/°C and of the storage side is 910 W/°C. The load is required for 8 h a day, from 8 am to 4 pm, and the air has a constant flow rate of 0.25 kg/s. The heat capacity of air is 1012 J/kg-°C and the flow rate of water through the load heat exchanger is 0.07 kg/s. For the period under investigation, the radiation and ambient temperatures shown in the following table apply. If the initial temperature of the storage tank on the day investigated was 42°C, estimate the energy supplied to the load and the amount of auxiliary energy required by the system to cover the load.

Time

Ambient temperature (°C)

I, (MJ/m2)

6-7

10

0

7-8

11

0

8-9

12

1.12

9-10

14

1.67

10-11

16

2.56

11-12

17

3.67

12-13

18

3.97

13-14

16

3.29

14-15

15

2.87

15-16

14

1.78

16-17

12

1.26

17-18

11

0

7.2 The system in Exercise 7.1 uses water at a temperature of 80°C instead of air. If the load is required for the same period of time, the flow rate of water is 0.123 kg/s, and the heat capacity of water is 4180 J/kg-°C, estimate the energy supplied to the load and the amount of auxiliary energy required by the system to cover the load.

7.3 Repeat Exercise 7.1 for an effectiveness of the load heat exchanger of 0.66 and compare the results.

7.4 Repeat Exercise 7.2 but the load is required for only 30 min of each hour of operation.

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