Why is important to select adequate outdoor conditions?

Outdoor condition selection is one of the preliminary design conditions we should determine. In many cases, the customer will fix them. In other cases, we must follow the regulations.

In the Pharmaceutical Industry, outdoor design temperature and relative humidity can have an impact on product quality. Quality departments normally assign a range for temperature and humidity that shall be monitored continuously within the production process (alert limits and alarm limits).

Consequently, an inadequate selection for the outdoor conditions can cause an out of specification report when conditions are out of the design.

This is a significant difference with respect to other HVAC systems as commercial or residential design. In these applications, outdoor design conditions are normally selected from tables (i.e. ASHRAE) or local regulations. As a result, this has a very important impact on equipment sizing and energy consumption.

The weather

In commercial and residential design is normally accepted to use a percentage of the occurrence. The most common are 0.4% and 1% for cooling. For heating, 99.6% and 99%.

See an example from the Cork location:

Source: http://ashrae-meteo.info

As you can see in the table, design condition for heating, 99.6% occurrence is -1.3°C dry bulb temperature. That means that 99.6% of the hours, temperatures will be above -1.3°C. The design condition for cooling is 21.2°C at 0.4% of occurrence. 0.4% of the hours, the temperature will be higher than 21.2°C.

As mentioned above, this 0.4% can origin a severe impact on the indoor temperature (more as the percentage of fresh air is increased). Let’s take a look at the worst-case scenarios:

The design

The same source offers us another important information: extreme annual design conditions. These temperatures and humidity are extremely unusual, but we normally consider any of these values for critical pharmaceutical HVAC design.

Note that the word critical. That means that other non-production areas, like offices, laboratories, CNC spaces, etc, using an occurrence percentage is common.

To sleep soundly at nights, for critical areas, the seasoned Pharmaceutical Engineer should choose an extreme annual design condition of 20 or 50 years return period value of extreme temperature. For other energy savings considerations, we shall establish and achieve a compromise with the quality acceptance range.

Look at this chart, showing graphically the effect in the energy used for cooling (enthalpy increasing) in our Cork example for the following design conditions:

Graphically you can see the difference between extreme and normal conditions in terms of energy:

As high are the points, higher is the enthalpy (energy per mass unit). And higher is the money of the sized equipment!.

As a conclusion, pharmaceutical HVAC outdoor design conditions have considerable consequences on our project!