HOW ARE YOUNG ENGINEERS BEING TRAINED IN THE HVAC INDUSTRY?

PART 3

Catalogues are designed to be a quick reference to illustrate a specific range of products and to specify an approximate unit performance rating at a specific location and operating condition, 
Catalogue engineering is sometimes applied to detailed designs and problems arise when the unit is commissioned and does not perform to the specification.  
The question is “why does this happen?” Why does the catalogue product not perform to its rating when commissioned for all locations within South Africa?
Performance ratings for catalogue products are generated with computer programs but are specified at certain nominal conditions. Therefore, the unit performance rating that is stated in a catalogue is a nominal performance rating and NOT the actual performance rating.

If you read the fine print at the bottom of the page, it will state that the nominal performance rating of a catalogue product is rated at sea level with an on coil temperature that is between 25°C to 29°C.

How often does comfort air conditioning system operate at these conditions? The answer is, “very seldom”.
So to understand where the difference in unit performance comes from, we need to understand the physics behind such a system.

The second law of thermodynamics states “that heat is transferred from a hot to a cold fluid”. The larger the temperature difference, the higher the heat transfer rate.
So if the refrigerant operates at 7°C and the return (on coil) air temperature is 26°C, there is a 19°C temperature difference. If the system now operates at 22°C, there is a 15°C temperature difference. When the temperature difference between the refrigerant and the return air is decreased, the performance of the unit also decreases.

Air conditioning systems for comfort cooling are set to maintain a room temperature of 22°C. By doing this, the performance of the system is reduced. To ensure that the correct unit for the correct application is selected, a larger unit that will provide a greater cooling capacity must be selected and installed.  
 Our previous article mentioned the importance of understanding basic principals in the field of mathematics and physics. This simple illustration explains the importance of understanding how HVAC systems operate and how catalogue engineering is different from detailed engineering.

So would you like to become a great engineer? First thing to do is always work on actual performance ratings. Always ensure that the unit has been sized for the correct application and the correct operating conditions of the site.

This same must be applied for estimating engineers as well. Tenders can be based and selected with units that are rated at a nominal capacity. This can be disastrous on any HVAC project as the submitted tender price will be too low and the unit will not perform once it is installed on site
 
To avoid these problems, ask the following questions and then submit the answers to the supplier when requesting the selection an HVAC cooling unit:
  • What altitude is the unit operating at?
  • What is the return air (on coil) temperature?
  • What ambient air temperature has the unit been rated at?

Once the supplier has submitted the unit performance rating and selected a cooling unit, check that the operating condensing temperature is between 45°C and 51°C.

Becoming a great engineer is to constantly ask the correct questions and empoyer yourself through personal development. Always question and challenge statements and the path to discovering the correct answers will be revealed with understanding the basic principals of mathematics and physics.
Wishing you a successful month ahead

ABOUT THE AUTHOR

Michael Young graduated from the University of the Witwatersrand in the field of Mechanical Engineering (B.Sc Mech Eng) in 2008. After he completed his undergraduate studies, he proceeded to work in the HVAC industry whereby he worked in the manufacture, contracting and consulting sector.  

In 2013, Michael qualified as a Professional Engineer (Pr.Eng) with the Engineering Council of South Africa (ECSA) and then founded M&Y Consultants and Trainers (PTY) Ltd whereby he is perusing his passion of helping others within the HVAC&R and Engineering industry through his training courses.

Michael is passionate about helping others and sharing his knowledge and experience that he has gained through working in the industry. 

He is a regular contributor for the Refrigeration and Air Conditioning Africa (RACA) Journal and has also presented technical talks in association with the South African Institute of Mechanical Engineers (SAIMechE), the South African Energy Efficiency Convention (SAEEE) and Data Centre Central Conference.