Professionals who care
HOW GEOGRAPHICAL LOCATION AND APPLICATION CAN AFFECT THE DESIGN OF AN HVAC SYSTEM : PART 5
By Michael Young ( Pr . Eng .)
MICHAEL YOUNG
Michael Young is a trainer , coach and mechanical engineer in the HVAC industry . He graduated from the University of the Witwatersrand in the field of Mechanical Engineering ( B . Sc . Mech Eng .) in 2008 and qualified as a Professional Engineer ( Pr . Eng ) in 2013 . Michael is passionate about promoting knowledge and helping other young engineers grow within the industry through his training workshops and coaching sessions .
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 . RACA
www . hvacronline . co . za RACA Journal I May 2021 43