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The solar heat absorbed at each time interval by the outdoor surface of the wall throughout the day goes thru this same process. Figs. 25 and 26 show the total solar heat flow during the second and third time intervals.

A rise in outdoor temperature reduces the amount of absorbed heat going to the outdoors and more flows thru the wall.

This same process occurs with any type of wall construction to a greater or lesser degree, depending on the resistance to heat flow thru the wall and the thermal capacity of the wall.

NOTE: The thermal capacity of a wall or roof is the density of the material in the wall or roof, times the specific heat of the material, times the volume.

This progression of heat gain to the interior may occur over the full 24-hour period, and may result in a heat gain to the space during the night. If the equipment is operated less than 24 hours, i.e. either skipping the peak load requirement or as a routine procedure, the peak load requirement or as a routine procedure, the nighttime radiation to the sky and the lowering of the outdoor temperature may decrease the transmission gain and often may reverse it. Therefore, the heat gain estimate (sun and transmission thru the roof and outdoor walls), even with equipment operating less than 24 hours, may be evaluated by the use of the equivalent temperature data presented in Tables 19

and 20.

Basis of Tables 19 and 20

- Equivalent Temperature Difference for Sunlit and Shaded Walls and Roofs

Table 19 and 20 are analogue computer

calculations using Schmidt’s method based on the following conditions:

1. Solar heat in July at 40_{° North latitude.}

2. Outdoor daily range of dry-bulb temperatures, 20 deg F.

3. Maximum outdoor temperature of 95 F db and a design indoor temperature of 80 F db, i.e. a design difference of 15 deg F.

4. Dark color walls and roofs with absorptivity of 0.90. For light color, absorptivity is 0.50; for medium color, 0.70.

5. Sun time.

The specific heat of most construction materials is approximately 0.20 Btu/(lb)(deg F); the thermal capacity of typical walls or roofs is proportional to the weight per sq ft; this permits easy interpolation.

Use of Tables 19 and 20

- Equivalent Temperature Difference for Sunlit and Shaded Wall and Roofs

The equivalent temperature differences in Tables

19 and 20 are multiplied by the transmission

coefficients listed in Tables 21 thru 33 to determine the heat gain thru walls and roofs per sq ft of area during the summer. The total weight per sq ft of walls and roofs is obtained by adding the weights per sq ft of each component of a given structure. These weights and shown in italics and parentheses in Tables 21 thru

33.

Example 1 – Equivalent Temperature Difference, Roof

Given:

A flat roof exposed to the sun, with built-up roofing,11_{2in. insulation,3 in.}

wood deck and suspended acoustical tile ceiling. Room design temperature = 80 F db

Outdoor design temperature = 95 F db Daily range = 20 deg F

Find:

Equivalent temperature difference at 4 p.m. July. Solution:

Wt/sq ft = 8 _{+ 2 + 2 = 12 lb/sq ft (Table 27, page 71)} Equivalent temperature difference

= 43 deg F (Table 20, interpolated)

Example 2 – Daily Range and Design Temperature Difference Correction

At times the daily range may be more or less than 20 deg F; the difference between outdoor and room design temperatures may be more or less than 15 deg F. The corrections to be applied to the equivalent temperature difference for combinations of these two variables are listed in the notes following Tables

19 and 20.

Given:

The same roof as in Example 1 Room design temperature = 78 F db Outdoor design temperature = 95 F db Daily range = 26 deg F

Find:

Equivalent temperature difference under changed conditions Solution:

Design temperature difference = 17 deg F Daily range = 26 deg F

Correction to equivalent temperature difference = -1 deg F (Table 20A, interpolated)

Equivalent temperature difference = 43 – 1 = 42 deg F

Example 3 – Other Months and Latitudes

Occasionally the heat gain thru a wall or roof must be known for months and latitudes other than those listed in Note 3 following Table 20. This equivalent temperature difference is determined from the equation in Note 3. This equation adjusts the equivalent temperature difference for solar radiation only. Additional correction may have to be made for differences between outdoor and indoor design temperatures other than 15 deg F. Refer to Tables 19 and

20, pages 62 and 63, and to the correction Table 20A. Corrections for these

differences must be made first; then the corrected equivalent temperature differences for both sun and shade must be applied in corrections for latitude. Given:

12 in. common brick wall facing west, with no interior finish, located in New Orleans, 30_{° North latitude.}

Find:

Equivalent temperature difference in November at 12 noon. Find:

Part 1. Load Estimating | Chapter 5. Heat And Water Vapor Flow Thru Structures

Solution:

The correction for design temperature difference is as follows:

Example 3, contd

Summer design dry-bulb for New Orleans = 95 F db (Table 1, page 11) Winter design dry-bulb for New Orleans = 20 F db (Table 1 page 11) Yearly range = 75 deg F

Correction in outdoor design temperature for November and a yearly range of 75 deg F

= -15 F (Table 3, page 19)

Outdoor design dry-bulb temperature in November at 3 p.m.

= 95 – 15 = 80 F

With and 80 F db room design, the outdoor to indoor difference is 80 – 80 = 0 deg F

Average daily range in New Orleans = 13 deg F (Table 1, page 11)

The design difference of 0 deg F and a 13 deg F daily range results in a –11.5 deg F addition to the equivalent temperature difference, by interpolation in

Table 20A.

Equivalent temperature differences for 12 in. brick wall in New Orleans at 12 noon in November:

∆tem for west wall in sun

= 7 (Table 19)-11.5 = -4.5 deg F

TABLE 19-EQUIVALENT TEMPERATURE DIFFERENCE (DEG F)