21 Sep Heat Gains
As discussed in the earlier post About Air Changes, we have to consider several aspects of the design of the air volume supply. One of them, and sometimes the most important, are the heat gains in the room. In this post, we are going to talk about particularities in the Pharmaceutical HVAC that differs slightly from other HVAC applications.
Usually, for HVAC calculations, we have to consider the contributions to the total heat gain of the building listed below. Nevertheless, many of them are less important than non-cleanroom HVAC design. This is because normally, a cleanroom is an enclosure inside a building. Like any building, this is in a certain way insulated.
The heat transmission through an enclosure can be expressed as:
Ht = Heat gain due to the transmission, kW
U = Overall heat transfer coefficient, W/m2degC
A = Wall area, m2
Delta T = Temperature difference, supply and return, degC
As per considered above, the transmission contribution to the total heat gain is very low. Normally, the surrounding areas of the classified rooms are conditioned as well. So Delta T will be low, and consequently, the heat will be too.
This will not exactly true for the lowest floor of the building (basement or ground floor, depending on the case). We should consider the insulation foreseen from the architects. We also should be careful with the roof. The interstitial space between the last floor and the roof probably will contribute to the transmission heat gain on that floor.
The most used insulation cores in standard cleanrooms are Rockwool and Polyurethane (PIR or PUR). The widths are normally between 40 – 60 mm.
Another commonly used cleanroom panel is the honeycomb type. Do not underestimate in the calculations because the U value for a 60 mm panel is around 5 W/m2degC. This is a not insulation material, and too high compared to a Rockwool 60 mm (0.35 W/m2degC).
2. Solar radiation through glass
This is a very important contribution to the total heat gain in residential and commercial HVAC applications. But not really common in a cleanroom as per the same reasons explained above for the transmission. Except when you have a singular project, you should not consider this contribution.
In this case, lighting has a significant impact on heat gain. Normally, production areas are designed to have more than 500-750 lux. Less perhaps in the case of corridors or gowning areas.
With traditional fluorescent tubes, you should consider 25-35 W/m2 or 5-15 W/m2 with LED.
For detailed calculations, you should get the input from your Electrical Department.
Chapter 18 of the ASHRAE Handbook – Fundamentals, can give you some good directions for the calculations.
There is nothing new under the sun with this contribution. You can use the same tables as per other HVAC applications. Do not forget to add the latent load!
In Chapter 18 of the ASHRAE Handbook – Fundamentals, you can find tabulated loads for different activities:
The heat gain from equipment is in many cases the main contribution to the total heat gain in a room. I strongly recommend getting this data from the manufacturer specifications. Pay attention if some equipment produces both sensible and latent heat.
Below there are some guidelines:
- Large locally heat-generating machines like welding stations, depyrogenation tunnels, may be interesting to remove the air just above heat generation instead of removing it from the HVAC system.
- In the same way, where large latent loads are generated, is preferably to extract the air instead of dehumidifying.
- Typical equipment with high heat generation in the Pharmaceutical industry:
- Packaging lines
- Depyrogenation Tunnels
- Unidirectional air flows
- Non-insulated jacketed vessels
- This list includes but it is not limited to the equipment installed inside cleanrooms. But do not forget other important sources of heat in technical spaces or plant rooms (heat exchangers, pumps, distillers, etc.)
- Do not forget to include the heat generated by the supply and/or return fans. Note that temperature can increase up to 2 °C!
- For motors installed inside the cleanroom, consider their rated power and the efficiency to calculate the released heat.
6. Final Considerations
Finally, I will finish this post with a few considerations, and tips.
- The diversity factor is not always applied. Airflow is always on to guarantee minimum air changes per hour. You can decrease airflow during non-production periods, for energy savings purposes, but remember to validate it!
- Apply a W/m² ratio to check consistency in calculations and discover mistakes.
- I recommend an allowance of 10% in sensible and latent heat calculations.
- If air changes define the airflow, and it supplies to several spaces, always check the supply temperature. This could do be necessary to use reheaters.
- Infiltrations? No, thanks. We will have them always under control, as we were discussing in the post Leakages Calculation
- If you think that the transmission, or solar radiation, is significantly important to the total amount of the heat gain, it could be recommendable to use dedicated software, like Hourly Analysis Program (HAP), by Carrier. https://www.carrier.com/commercial/en/us/software/hvac-system-design/hourly-analysis-program/
- The using of non-specific pharmaceutical HVAC software like HAP could be very interesting to perform an energy analysis of the plant. But we will discuss this in another post.