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Peak cooling coil loads can often occur during either the Monthly Design Dry-Bulb conditions or the Monthly Design Wet-Bulb conditions. As a continuation of ASHRAE Heating and Cooling Loads and HVAC Equipment Sizing, this article is tailored to provide guidance to engineers designing in humid climates. The 9,237 global locations available in IESVE Software uses the ASHRAE Design Weather Database (Version 7.0). These locations datasets can be used with in two ways to calculate cooling loads under design conditions.
| Peak Dry-Bulb Temperature with Mean Coincident Wet-Bulb Temperature | Peak Wet-Bulb Temperature with Mean Coincident Dry-Bulb Temperature |
| Current default setting in IESVE Software | Global Presets - Download available here (18 MB). |
The capacity of a cooling coil is determined by the amount of heat it extracts from the airstream. Often, the leaving air conditions of a cooling coil are set at a constant temperature, however the entering air conditions can vary depending on the ventilation to return air ratio.
Figure 1: Mixed Air Diagram
The psychrometric property, enthalpy, is the amount of heat per unit mass of a volume of air. The peak load on the cooling coil occurs at the maximum difference between the entering and leaving air enthalpies.
Figure 2: Cooling Coil Performance
The maximum difference in Enthalpy can occur either on Monthly Design Dry-Bulb or Monthly Design Wet-Bulb conditions. The Design Wet-Bulb conditions can often be overlooked in dry-climates, however, in humid climates (climate zones with the index “A”) the wet-bulb design days must be evaluated. The map below shows the areas where evaluating monthly wet-bulb design days is recommended.
Figure 3: Global Humid Climates
ASHRAE weather datasheets provide both monthly design weather data conditions. Selecting a location in IESVE’s ApLocate tool will import the hourly design day conditions per month. The engineer must use their best judgement to determine if cooling load calculations shall be conducted with wet-bulb design criteria; if in doubt, test both conditions.
Figure 4: Monthly Climatic Design Conditions
Locally overwriting the design weather data to match monthly design wet-bulb conditions is easy to do in ApLocate, as shown below.
Figure 5: Selecting and Overwriting Weather Data
The system reports generated from a System Equipment and Plant Sizing Calculation show the peak cooling coil performance data. As observed in the report below, the wet-bulb conditions result in a cooling coil capacity approximately 10 tons higher, or ~16% higher. This test was conducted in Houston, Texas (Climate Zone 2A) with 30% outdoor air.
Figure 6: Comparing Cooling Coil Capacity using Reports
Checking system sizing data in VistaPro is a powerful feature of IESVE. To ensure accurate comparisons, it is essential to name the calculations appropriately with the tested conditions; otherwise, there is a risk of overwriting previous results needed for comparison. In the figure below, we plotted the total cooling coil load for the “cooling season” design days: May through September. Notice the calculated total cooling load at the hourly wet-bulb design day conditions per month are much higher than that of the hourly dry-bulb design day conditions.
Figure 7: Total Cooling Coil Capacity reviewed in VistaPro
IES created a downloadable library of 9,237 files (18 MB) using the ASHRAE Design Weather Database (Version 7.0) that can be downloaded to imported into your project. Unlike the default setting in IESVE, this dataset will represent the Peak Wet-Bulb Temperature with Mean Coincident Dry-Bulb Temperature. Select the location file you desire to import and copy it to the following file path in your local directory:
C:\ProgramData\ies\apps\LocationPresets
Once copied, restart IESVE software and navigate to IESVE’s ApLocate to import the hourly wet-bulb design day conditions. See the example workflow below:
Figure 8: Importing a Design Weather Preset
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