Analysis of Summer Heat Removal in the "12x12 units", Beijing

Guilherme Carriho-da-Graca, MIT Building Technology Program, gcg@mit.edu

March 11, 1999

Below a simple analysis of stack effect ventilation for a six story housing unit is presented. Air change rates are calculated as well as predicted temperature increase between the inlet and outlet. A comparison of the pressure generated by wind the stack is also presented.

In the figure on the right we can see a representation of the way the building is being modeled. Apertures only exist in the upper and lower floor and the internal; resistances are neglected. The internal flow resistances are neglected. The lower and the upper floor have the same opening area. Once the air enters a unit it is considered to suffer total mixing and an immediate increase in temperature due to the internal gains. In the charts presented below the number of units is a parameter, when a lower number of units is considered the lower aperture is placed in the lowest unit and the higher in the highest unit.

The values proposed by ASHRAE were used for estimating the internal gains. Due to the fact that we are studying apartment units we use a metabolic rate for each occupant of 1.2 met , which for an exposed area of 1.2 m2 gives a heat gain of 85 W per occupant. Each unit is considered to have five occupants. The machine and appliances heat gains are considered to be 470 W (single family house). Therefore for each of the units we have:

Total Gains=470+5x85=895 W

The ceiling height used for each floor is 2.5m.

Figure 1, Aperture configuration.

Results

Figure 2

Temperature increase of the air as in each unit, for a range o aperture areas. Upper curve is for two stories, lower curve is for six stories.

Figure 3

Air changes per hour in each building, for a range of aperture areas.Upper curve is for two stories, lower curve is for six stories.

Figure 4.1

Comparison between the stack generated pressure and the wind generated pressure at the outlet (see blue arrow in figure 1) for a two stories building. The aperture area used for the calculation of the required heat gains per unit (horizontal axis) was 1.5 m^2. The wind velocity used was 1.5 m/s.

Figure 4.2

Comparison between the stack generated pressure and the wind generated pressure at the outlet (see blue arrow in figure 1) for a three stories building. The aperture area used for the calculation of the required heat gains per unit (horizontal axis) was 1.5 m^2. The wind velocity used was 1.5 m/s.

Figure 4.3

Comparison between the stack generated pressure and the wind generated pressure at the outlet (see blue arrow in figure 1) for a four stories building. The aperture area used for the calculation of the required heat gains per unit (horizontal axis) was 1.5 m^2. The wind velocity used was 1.5 m/s.

Figure 4.4

Comparison between the stack generated pressure and the wind generated pressure at the outlet (see blue arrow in figure 1) for a five stories building. The aperture area used for the calculation of the required heat gains per unit (horizontal axis) was 1.5 m^2. The wind velocity used was 1.5 m/s.

Figure 4.5

Comparison between the stack generated pressure and the wind generated pressure at the outlet (see blue arrow in figure 1) for a six stories building. The aperture area used for the calculation of the required heat gains per unit (horizontal axis) was 1.5 m^2. The wind velocity used was 1.5 m/s.

Comments on the results:

Figure 2 shows as expected that the smaller the area the smaller the flow rate. Since the heat to be removed is constant (the internal gains) the temperature difference between the inlet and the outlet air is inversely proportional to the aperture areas. Note that, for example, when five units are considered the internal gains are only from the five units (that is the reason why the temperature difference decreases with the number of units).

Figure 3, in this chart it is important to note that the air change rate is for the whole building.

Figure 4, In this figure we compare the stack pressure with the wind pressure at the outlet (see blue arrow in figure 1, we are comparing the pressure generated by wind that comes from left to right, generating a pressure that works against the stack effect).

Pstack depends on the square root of the internal gains, therefore the ratio:

Pstack/Pwind increases with the internal gains.

Whenever Pstack>Pwind the stack will still remove the internal gains (although with the wind pressure working against the stack the building will heat up because the air change rate will decrease).

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