By Michael Young (Pr.Eng)
The effects of climate change on the HVAC system of tomorrow…
HVAC systems maintain comfort while ensuring that indoor air quality is maintained within allowable limits as per international or local standards.
In order for both these criteria to be met, the HVAC system must be able to operate in high ambient temperature climates as well as provide enough dilution of airborne contaminants through the introduction of fresh air into the system.
The increasing threats of global warming and climate change have changed the ambient air conditions and on-coil conditions of newly designed HVAC systems. So, what do I mean by this statement?
The performance of a DX or air-cooled water chiller is impacted by the ambient air condition onto the condenser. The higher the ambient air condition, the lower the performance of the system at a constant condensing pressure.
To explain this in simple terms, let’s imagine that the condenser within our DX system must reject a THR of 50kW and is designed to operate at an ambient temperature of 35⁰C and a condensing temperature of 50⁰C. Mathematics tells us that this particular system is designed to operate with a 15⁰C temperature difference.
If the ambient temperature rises to 37°C, the temperature difference between the ambient air condition and condensing temperature is now reduced to 13°C. The law of thermodynamics tells us that if we decrease the temperature difference between the two fluids, we ultimately decrease the heat transfer.
A decrease in heat rejection from the condenser will cause other problems within the system such as insufficient cooling of the compressor motor windings or insufficient cooling of the air within the space.
To counteract this effect, two options may be performed:
- The condensing pressure can be increased to counteract the ambient temperature increase. This option does however cause other problems such as reduced system capacity as well as increased energy consumption. Increasing the condensing pressure reduces the density of the refrigerant which causes its thermal properties to decrease.
- Since the compressor pressure is increased the electrical consumption of the compressor also increases which causes a reduction in the system`s efficiency. This option may be easily performed by adjusting operating setpoints on the system but the knock-on effects are negative.
- The second option is to increase the size of the condenser within the initial design of the HVAC system. To accomplish this, the condenser coil may either have a greater number of rows or a smaller fin spacing. Other constructional changes may be a higher number of fans to increase the overall airflow through the condenser coil as well as a longer sized condenser coil.
A larger condenser THR capacity means that the system is still able to reject the heat to the ambient air when ambient conditions increase. This option increases the overall capital cost of the project and also requires more space on site for the placement of the larger condensers.
This option is generally the new trend but caution has to be exercised during the tender phase as all condensers need to be rated at the same ambient conditions. Inconsistent ambient temperatures cause the final costs to vary which affects accuracy when performing cost and performance analyses during the tender phase of a project.
As you can see, ambient conditions have a greater impact on the performance and design of an HVAC system and the goal is try to reach that optimal design point whereby the system is not over- or under-designed.
Join us next month where we tackle the impact of the on-coil conditions within the system.
Wishing you a successful month ahead, and chat soon.