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Home » The easiest way to kill the performance of a DX cooling system – Part1

The easiest way to kill the performance of a DX cooling system – Part1

By Michael Young, Pr. Eng.

Part 1: blindly trust the computer results.

The world that we live in has evolved over the past 20 years. What used to take us years to complete is now done within a matter of hours and this is all thanks to the invention and implementation of computers.

Now, would it be possible to say that while computers have made life simple and more efficient, they have also opened the door for errors to be easily made?

Think about it, to select a cooling unit, all we need to do is input some variables, click on the select button and bam! a unit is selected for us without even thinking.

What about doing the design of an AHU unit? We input some variable and bam!, the AHU is designed, selected, costed, and even drawn in 3-dimensions within a matter of seconds.

But what if I told you that while these systems have been fantastic in helping us design systems more efficiently, these computer systems/programs are not fool-proof and are prone to error. Think about it, have you ever questioned the results from a computer program?

How do you know the computer program results are even accurate? Have you ever had to insert some crazy inputs into a program and get a crazy result as an output?

I had this experience where I had to select a cooling unit with a return air condition of 37⁰C / 50% RH, and I had to increase the unit size to get to the net sensible cooling load.

In reality, it’s highly unlikely that air will be returning to the cooling unit at this condition. So, I contacted the client and learnt that a typing error was made on the specification document and that an addendum was to be shortly issued with the correct return air conditions.

Now don’t get me wrong, computers are awesome, but they are just a tool. Quality and accuracy come with being able to use these tools correctly and by having a good understanding of the physics being applied.

Let’s talk about refrigerant pipe design as an example. There are tons of tools out there and some are even free and are available online. Now if you leave the selection on “auto”, the program will size the correct pipe size for you. However, when it comes to sizing the suction line of a DX system, the system has two options, namely a horizontal run and a vertical run selection.

Now why would we have two different options for the same line? Is it insane to install two different refrigerant pipe sizes on the suction line? What are the implications if we get this pipe sizing wrong? Do we size the piping for maximum or the minimum load?

Unfortunately, no computer program will tell you this answer but if you join us in next month’s publication, we will share with you the procedure on how to size refrigerant piping without blindly trusting the computer results.

Wishing you a successful month ahead and chat soon.

About Michael Young

Michael Young, Pr. Eng.

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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 engineer grow within the industry through his training workshops and coaching sessions. Michael can be contacted on michael@myhvaccoach.com or 073 171 2311 for any questions or HVAC training needs.

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