COMPUTER SIMULATION OF COOLING LOADS
4.2.2 ZONEST
ZONEST is the utility in which the simultaneous cooling load profile for many combined zones is calculated. Each ".zpa" file is selected, one by one, and placed in a selection set. A useful trick at this stage, is the ability to save this selection set as a separate file, with a ".zst" file extension. This negates the need to re-select each ".zpa" file manually to perform the same calculation again, at a later stage.
There are two outputs from the cooling load Calculation. The first is a tabular summary of the cooling load for the design day of each month. The second page is a graphical display of the cooling load for the design day of the year, that is the day in which instantaneous cooling load reaches the maximum value.
An important factor to note is that the heat load profile may be shown for an operating schedule of 12, 16 or 24 hours. This means that the cooling load graph will have a corresponding domain.
4.2.3 The WEATHER UTILITY
The WEATHER UTILITY is used to specifY the outdoor design conditions for the region in which the building is located. A weather file consists of a two page summary. The first page gives details such as town name, altitude, dry and wet bulb temperatures for the design day, the daily range, as well as the latitude of the town.
The second page gives values of temperature and humidity ratio corrections to be used for different times of day, and different months of the year. This allows the user to construct a temperature and humidity ratio profIle for each month of the year. The WEATHER UTILITY includes existing files for almost 40 towns and cities in South Africa. Itis also possible for the user to create new weather files for towns. Weather files are saved with a ".twn" file extension.
4.3 Weather data used in LOADEST
As was mentioned in 4.2.3, the weather data in LOADEST is represented by a series of dry-bulb temperature and humidity ratio pairs. These are obtained by means of a peak design value, with corrections applied for each month, and hour of the day. These allow the state point for each hour of the year to be plotted on a pychrometric chart.
At the moment all that will be discussed is the method of obtaining each of these state points.
The accuracy of this means of representing weather data will be shown later.
4.3.1 Dry bulb temperature data for Durban
The outdoor design peak for Durban is29.So
e
dry-bulb temperature. The hourly and monthly corrections are shown below. The corrections have units ofoe. The corrections used by Loadest are shown in Tables 4.1 and 4.2.The monthly and hourlydrybulb temperature corrections were applied to the peak design value of29.Soe. The hourlydrybulb temperatures for the year are shown in Appendix I
Time Correction[0C] Time Correction[0C] Time Correction[0C] Time Correction[0C]
01hOO -6.3 07hOO -5.3 13hOO 0.0 19hOO -3.7
02hOO -6.7 08hOO -3.6 14hOO -0.1 20hOO -4.2
03hOO -7.0 09hOO -1.7 15hOO -0.7 21hOO -4.6
04hOO -7.2 10hOO -0.9 16hOO -1.5 22hOO -4.9
05hOO -7.4 11hOO -0.5 17hOO -2.3 23hOO -5.4
06hOO -6.8 12hOO 0.0 18hOO -3.0 24hOO -5.7
Table 4.1: Hourlydrybulb temperature corrections used in LOADEST
Month Correction[0C] Month Correction~C] Month Correction[0C]
January 0.0 May -3.0 September -2.5
February 0.0 June -5.5 October -1.0
March 0.0 July -6.1 November 0.0
April -1.0 August -5.0 December 0.0
Table 4.2: Monthlydrybulb temperature corrections used in LOADEST
4.3.2 Humidity ratio data for Durban
No peak design humidity ratio is given in the weather files. Rather, the peak design wet bulb temperature is given. For Durban this value is 25.5°C. By plotting this value of wet bulb temperature, together with the peak design dry bulb temperature on a psychrometric chart, the peak humidity ratio was obtained. This value is 199/kg. There are no hourly corrections for hwnidity ratio, only monthly corrections. The validity of this method will be discussed later.
The humidity ratio corrections are shown in table 4.3 for each month, in units of g/kg. The corrected values of humidity ratio are shown in Table 4.4 for each month, in units ofglkg.
Month Correction Month Correction Month Correction Month Correction
January 0.0 April 0.0 July -11.0 October -1.0
February 0.0 May -4.0 August -7.0 November 0.0
March 0.0 June -8.0 September -3.0 December 0.0
Table 4.3: Monthly hwnidity ratio corrections usedinLOADEST
Month Humidity ratio Month Humidity ratio Month Humidity ratio Month Humidity ratio
January 19.0 April 19.0 July 8.0 October 18.0
February 19.0 May 15.0 August 12.0 November 19.0
March 19.0 June 11.0 September 16.0 December 19.0
Table 4.4: Monthly humidity ratio data used in LOADEST
4.3.3 Internal design conditions
The conditions to be maintained inside the conditioned space are just as important as the external conditions. The internal desigu conditions are usually fairly standard for comfort applications, but may be more varied in applications where certain conditions must be maintained for industrial processes.
The standard internal design conditions for Durban in LOADEST are 22.50C dry bulb temperature and 50% relative humidity. These values are suitable for all the areas considered in this study, and will therefore not be altered.
I
Create new town\ file
I
Existing town files saved with the eh.'tension
".two"
Files saved with the extension
".inp"
WEATHER UTILITY
Export town files to LOADEST
Create geometry
..- ~..LIr _____"
r-L-
...J~
LOADEST~
. . . - - -..--1 ~
Select design conditions ,Ir
Save :file for zone peak with extension ".zpa"
Other "zpa" files saved from
previous projects
..
ZONESTSave selection set with a extension of
".zpa"
Hourly summary for the year
U
Design day peak graph.---~ I __
Calculate zone peak _
'---IlL
Figure 4.1: Algorithm for the use of LOADEST