What is the criteria of selection of package air conditioning units?

The following criteria to be ensured during selection, installation and commissioning of Packaged air conditioning equipment:

1) Capacity not to exceed 15 Sqm of floor area / Ton for central unit from 20 to 50 Tons, and 10 to 12 Sqm/Ton for units below 20 Tons.

2) The airflow rate to be fixed 400 CFM/Ton plus or minus 5%.

3) Limited to maximum 4 floors high, generally upto two floor.

4) Check for the ventilation space requirement for equipment installated at outdoor as manufacturer's recommendation.

5) For use in gulf countries the selection of nominal tonnage should be based on 46 degree celsius, and during operation the unit components should withstand 50 degree celsius. Also the SHR ratio to be 0.7.

6) Select the capacity 15% more than the design capacity.

7) Unit to conform to AHRI/Eurovent standard for construction quality and testing.

8) The blower fan and condenser fan to be AMCA certified.

9) Aluminium prefilter & Synthetic pleated / panel filters with MERV 8 standard is advisable for better air quality. The filter to be easily accessible for maintenance.

10) Duct design to be taken care, velocity not to exceed 1300 FPM for capacity 20 - 50 Tons, 1000 FPM for 6 to 20 Tons.

11) 20% additional ESP values to be added on the calculated for blower motor capacity selection.

12) Always select proper sound attenuator in supply and return ducts for units from 6 to 50 Tons to control the room ambient noise less than 35 DB.

13) While connecting the supply air duct to equipment supply air outlet, ensure that a minimum of 1 meter of straight duct piece of the size of the supply air outlet is connected between them. This is a proper air distribution design and installation, the result shall be unnecessary noise shall be avoided, air flow rate and ESP ratings installed shall be achieved 100%.

14) BMS monitoring and control with communication protocol between the machine and the PC workstation, for a better control of room temperature and humidity as well as easy and cost saving maintenance of the equipment.

Since the unit is installed outdoor, the factors such as high ambient temperature, rapid air filter choking, corrosion of unit components results in the deficiency of unit performance. Hence all the above interpretations should be keenly taken into consideration. 

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Based on my experience the CFM/TR ratio of the package units are usually 400  CFM/TR, while as per Cooling Load calculation we usually get less than 400  CFM/TR. I would suggest following two things:

1. Always select the unit on the basis of actual capacity i.e., tonnage of the equipment at peak temperature of the location. Don't use tentative capacity, since equipment de-rate at high temp.
2. At the time of selection don't base on airflow, equipment must be selected based on capacity.