Regional News
Application engineering 'key to good air distribution'
01 August 2002
I refer to the article 'Bigger focus urged on air distribution' in Gulf Construction's May 2002 issue and as a 1971 graduate engineer specialising in the field of air-conditioning for over 30 years - having worked in Bahrain, Cyprus and Australia over the last 27 years - I have the following comments to make:
The published article appears to be an attempt by a manufacturer to denigrate competition rather than an unbiased attempt based on sound technical principles to inform the user. One may even contend that the article tries to mislead by relating sick building syndrome to air distribution products without a proper analytical technical link and making comments like "UAE has learnt by making mistakes which Qatar can avoid" (read by using a particular brand of products??).
Achieving optimum air distribution is an application engineering exercise and is quite different from product design and manufacture. Consequently, the quality of air distribution, let alone healthy/sick building syndrome etc, are not the preserve of the air distribution product manufacturer as the article seems to imply. Just because a grille is manufactured by "A" and tested by "B" does not guarantee either better air distribution or a healthy building as compared to another grille manufactured by "X" and tested by "'Y". The relationship is akin to that between the cooking ingredients (product) and the recipe (application) or the cloth (product) and haute couture (application).
Of course, there definitely is a contribution to be made by the product manufacturer in terms of design features, finish, versatility of application etc, just like better vegetables will make a better salad. But the art is more in the application. The undersigned is supremely confident of designing optimum air distribution systems whether it be using products of Anemostat or Air Master, Titus or Trox. Every manufacturer's product would vary in shape, size, dimension (that is why the competition lives) and will have slightly different performance.
The difference in performance is inevitable even with a single make of product. The same slot diffuser of make "A" will have a different performance based on room length to opposite wall, ceiling heights etc.
Regarding, the testing laboratory, ETL/ITS carry out testing in accordance with. ANSI, ADC codes and not arbitrarily. A competent application engineer should be able to evaluate the data with reference to the basis of that data and apply accordingly.
Testing of air distribution devices under isothermal conditions is recommended rather than at a fixed differential of 11 deg C as stated in the article. The application engineer can then carry out calculations for drop (cooling) or rise (heating) depending on the actual temperature difference and characteristic data of the air distribution device. Reputed manufacturers like Trox publish data in graphical form for determining drop and rise. Incidentally, the11 deg C differential is not mandatory as the article suggests but rather a maximum allowable limit based on psychometric realities. In fact, lower differentials are often necessary and achieved, coincidentally for reasons of better air distribution.
It would be enlightening to know if Dr Bilasy's conclusions on sick building syndrome were related to application engineering aspects of air distribution or directly related to the make of the product.
It should be borne in mind that new designs of air distribution devices are being developed especially from North European manufacturers. Therefore, manufacturers' following designs of Anemostat, Tuttle & Bailey or Titus cannot rest on their laurels alone and claim sole title to technically correct products.
The article attempts to relegate a secondary role to application engineering. A good application engineer can achieve good air distribution out of any well-made product albeit with different performance characteristics.
DRM Pai
Chief design engineer, Mechanical Projects
GP Zachariades (Overseas)
Bahrain
* The author replies ...
I take this opportunity to congratulate Mr Pai and accept his specialisation skill in the field of air-conditioning. However, competent engineers always analyse all technical aspects and assert their authority while making fair evaluations.
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A: Features: a) Suitable for heating/cooling; b) Uniform airflow; c) Right, left and downward air movement; d) Pattern control blade
B: Features: a) Heating only; b) Turbulent airflow; c) Clockwise/anti- clockwise air movement; d) Downward thrust; e) Deflector blade |
The diagrams A and B are worth a thousand words. It should be noted that with a slot diffuser of type B, the design creates a clockwise/anticlockwise air movement within the system and will not permit a uniform airflow nor meet the conditions of data published in the catalogue.
I hope these diagrams are self-explanatory. Specific designs will accomplish specific tasks only. Interestingly, ETL safety products logo and/or ISO certification, coupled with attractive prices, have been used to establish a market for these designs.
Comfort applications in this part of the world require air distribution of about 2 cfm/sq ft. Data obtained by using a pitot tube does not comply with BS 4773, therefore are inaccurate. Accurate data presented in a simple format will benefit all.
Let's wake up to hard facts - technically correct products will enhance the capabilities of an application engineer while those with characteristics that do not suit the application will cause problems for sure. As technocrats, we are all responsible for ensuring a well-performing air distribution system, in all respects.
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