Sizing - Air Conditioner Sizing - Part II


This articles talks about air conditioner sizing, using psychrometric chart, comfort, and duct sizing charts, in conjunction with Manual J. General checklist is provided for the process.

By TS Mohamed Rasied

AC sizing is a long topic, and hence I have broke the articles into two parts.

The first part of this article talked about the fundamentals and concept of AC sizing, sizing using square feet method and Manual J.

Part II of air conditioner sizing will talk about the integration of three different charts, and Manual J, in AC sizing. We'll begin now.

Comfort, Psychrometric and Duct Sizing Charts' Use in AC Sizing

This method of air conditioner sizing is more extensive than using Manual J alone, and it is especially useful in sizing for central air conditioning units - where you'll have multiple rooms to cool, with fresh air intake from outside.

Here's the checklist for AC sizing using this method:

  1. use comfort chart, to set the desired indoor conditions.

    Comfort chart is formulated by ASHRAE, and shows a range of comfort levels for human being.

    The comfort levels are provided in terms of humidity, and temperature

  2. next, Manual J is used to find out the sensible, and latent heat gain and loss in each room within a desired building.

    Room slope will be assigned for all rooms, individually.

    It is carried out by dividing sensible heat, to total heat (sensible, plus latent heat)

  3. now, psychrometric chart is to be used. This chart, will eventually determine the minimum air conditioner size for your building.

    The size is determined by finding out the following values,

    • mass flow rate of air into each room,

    • fresh air to recirculated air ratio,

    • and finally, the air conditioner size from cooling load value

    Data that you will need include, the volumetric air requirement per person, of 5 litres/person/compartment/second, outside, and inside temperature and humidity, and room slope values

  4. ducts will be the final item to be sized, as it needs information of air mass flow rate.

    Duct sizing charts need to be used for this, with prior information of maximum air draft at design condition, and duct resistance values

The checklist above will help you in performing air conditioner sizing, using psychrometric, duct sizing, and comfort charts.

It is quite an extensive process, but worth the effort for central air conditioner sizing.

TS Mohamed Rasied,
Provides information on air conditioners and air conditioning in her site,
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