By Michael Young (Pr.Eng)
Why on-coil condition is so important.
Have you ever wondered why most suppliers have a small note that states something along the lines of “the above unit capacities are based on 0m altitude and 24⁰C db / 50% RH on-coil or return air condition” under the unit performance rating? Why is this note so important, and what does it mean?
I have seen the mistake made so many times where nominal cooling ratings are taken as rated cooling capacities. So, what is the difference between these two ratings?
A nominal rating is where a cooling unit’s capacity is based upon a specific standard and the chosen standard is based on certain test conditions which give acceptably uniform results across all suppliers.
So, the term ‘nominal’ is generally geared towards catalogue equipment that is used in a commodity market. An example of this would be a 12 000 Btu/hr mid wall split unit that has been stated as such in a catalogue, and which most suppliers usually stock.
Rated capacity is the actual performance of a cooling unit at a specific set of conditions that may differ from that which was tested and certified. So, here is an example. Let’s imagine that we require 12 000 Btu/hr of cooling at 23⁰C/ 50% RH at an altitude of 1 400m.
If we now re-rate the cooling at these conditions, you will notice that the capacity of the cooling unit decreases. So, why does this happen?
In a previous article we discussed that if we reduce the temperature difference between two substances, the heat transfer also decreases. So, by reducing the return air temperature from 24⁰C to 23⁰C, we have essentially reduced the temperature difference between the return air and the refrigerant by 1⁰C. The other factor is altitude.
Have you ever noticed that you may have more energy in coastal areas than up at high altitudes? The main reason for this is that there is much more air down at the coast than there is at higher altitudes. The parameter that is used in air conditioning to describe this variation is air density.
So, when at higher air density, we essentially move more kg/s of air through the cooling unit. More mass-flow of air means more cooling capacity.
As we can see, the rated cooling capacity varies according to the site location as well as the return air temperature of the cooling unit. Besides the initial selection of the cooling unit, why else is on coil (return air) condition so important?
I had one instance on site whereby the cooling unit was not delivering the supply air temperature as per the original design. I checked the selection and everything looked okay on the design side. It was later discovered that a damper in the return air ducting was left open and the system was drawing ambient humid air into the system.
This essentially caused the cooling to perform more latent cooling therefore it was unable to deliver the design supply air temperature. The important lesson that I learnt was always to ensure that the unit is commissioned as per the original design conditions.
Communication is important. If the correct design conditions are not conveyed to the commissioning team, the unit may be started with the factory default settings and the system may not perform as per the design requirements.
So that’s a wrap on this series on how geographical location can influence the design of an HVAC system. It was great to share these articles with you so join us next time as we will be providing you with more great content in the HVAC field.
Wishing you a successful month ahead and chat soon.