- Category: Training Features
- Published on 01 July 2016
- Hits: 272
By: Michael Young (Pr. Eng.).
Cataloguing can be useful but, if you want to become a great engineer, you can’t take things at face value.
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, but problems arise when the unit is commissioned and fails 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 stated in a catalogue is a nominal performance rating, 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 and 29°C. How often does a 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, then 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 principles 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? The first thing to do is to 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.
The same must be applied for estimating engineering as well. Tenders can be based upon 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, pose the following questions to the supplier when requesting the selection of 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.
Part of becoming a great engineer is to constantly ask the correct questions. By always questioning and challenging statements and by understanding the basic principles of mathematics and physics, the path to discovering the correct answers will be revealed.
The time to take action is now to ensure that you don’t become a catalogue engineer.
To receive more free tips and a free consultation on how to avoid becoming a catalogue engineer, visit us at www.mike‑young.com.
Wishing you a successful month ahead.
- The time to take action is now to ensure that you don’t become a catalogue engineer.
- Why does the catalogue product not perform to its rating when commissioned for all locations within South Africa?