6.1 A building has a peak heating load equal to 18.3 kW and a peak cooling load of 23.8 kW. Estimate the seasonal heating and cooling requirements if the heating degree days are 1240°C-days, the cooling degree days are 980°C-days, the winter indoor temperature is 23°C, and the summer indoor temperature is 25°C. The design outdoor temperature for winter is 2°C, and for summer, it is 39°C.

6.2 Estimate the overall heat transfer coefficient of a wall that has the following layers:

Outside plaster, 2 cm. Brick, 20 cm. Air gap, 2 cm.

Polyurethane insulation, 3 cm. Brick, 10 cm. Inside plaster, 2 cm.

6.3 Estimate the overall heat transfer coefficient of the wall in Exercise 6.2 by replacing the 10 cm inside brick with the same thickness of medium-density concrete.

6.4 Estimate the overall heat transfer coefficient of the wall in Exercise 6.2 by replacing the air gap and polyurethane with 5 cm polyurethane.

6.5 Estimate the U value of a pitched roof that has a ceiling U value = 1.56 W/m2-K, area of 65 m2, and a roof U value = 1.73 W/m2-K. The slope angle of the roof is 35°.

6.6 A building has a south-facing thermal storage wall with night insulation of Rins = 1.35 m2-K/W, applied for 6 h. Estimate the monthly heat transfer through the wall into the indoor space with and without night insulation for the month of January. The following data are given:

Uo = 6.3 W/m2-K. w = 0.31 m. k = 2.2 W/m-K. hA = 8.3 W/m2-K. Ht = 11.8 MJ/m2-K. (to) = 0.83.

Tr |
= 21°C. |

Ta |
= 3°C. |

Aw |
= 25.1 m2 |

6.7 A house has a south-facing window of 1.8 m height, located in 35°N latitude. The overhang is wide enough so as to neglect the side effects and its length is 0.9 m located 0.6 m above the top surface of the window. Estimate the shading fraction for a vertical window facing due south and a window in the same direction and tilted 10° from vertical at 11 am and 2 pm on July 17.

6.8 A building with a south-facing window with height = 2.5 m_and width =_5 m is located in an area where KT = 0.574, hss = 80°, Rb = 0.737, H = 12.6MJ/m2, and Fw = 0.705. The ground reflectance is 0.3. Estimate the mean monthly area-average radiation received by the window when there is no shading and when an overhang with a gap of 0.625 m, extension 0.5 m on both sides of the window, and projection of 1.25 m is used.

6.9 A solar space and hot water heating system has a collector with FrUl = 6.12 W/m2-°C and an area of 20 m2. The flow rate in a collector-storage heat exchanger of antifreeze and water is 0.02 kg/s-m2, the heat exchanger has an effectiveness equal to 0.73. What is the ratio FR/FR if the cp of water is 4180 J/kg-°C and that of antifreeze is 3350 J/kg-°C?

6.10 A room is maintained at a temperature of TR = 22°C and has a (UA) = 2850 W/°C. The ambient temperature is 2°C and the storage tank temperature is 75°C. Estimate the space load, domestic water heating load, and rate of auxiliary energy required for the following conditions:

Heat exchanger effectiveness = 0.75.

Flow rate of air side of heat exchanger = 0.95 kg/s.

Environmental temperature at the space where storage tank is located = 18°C.

(UA) of storage tank = 3.4 W/°C. Mass flow rate of domestic water = 0.15 kg/s. Required domestic water temperature = 55°C. Make-up water temperature = 14°C.

6.11 A liquid solar heating system has a 16 m2 collector and is used to preheat the city water, which is at a temperature of 12°C. If the tank is fully mixed and the capacitance of the collector side of the heat exchanger is 890 W/°C and the storage side is 1140 W/°C, estimate the final temperature in the storage tank at 3 pm and the energy balance of the system for the following parameters and conditions of the system:

Heat exchanger effectiveness = 0.71.

Storage tank capacity = 1100 L.

Initial tank water temperature = 40°C.

Environmental temperature at the space where storage tank is located = 18°C.

The meteorological conditions and load flow rate are given in the following table.

Hour |
I, (MJ/m2) |
Ta (°C) |
Load flow rate (kg) |

9-10 |
0.95 |
13 |
160 |

10-11 |
1.35 |
15 |
160 |

11-12 |
2.45 |
18 |
80 |

12-13 |
3.65 |
22 |
0 |

13-14 |
2.35 |
23 |
80 |

14-15 |
1.55 |
21 |
160 |

6.12 Using the data of the previous problem, estimate the effect of increasing the effectiveness of the heat exchanger to 0.92 and the city mains water temperature to 16°C. Each modification should be considered separately and the result should be compared to those of the previous problem. In every case, the energy balance should be checked.

Was this article helpful?

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

## Post a comment