How to Calculate the Capacity of Air Handling Units?

An Air Handling Unit (AHU) is a common HVAC device used to control temperature, humidity, and to remove or reduce air pollutants during both hot and cold seasons. AHUs are widely used in industrial, commercial, residential buildings, hospitals, auditoriums, hotels, halls, cinemas, and terminals, and are produced in various capacities based on project requirements. The capacity of an AHU refers to the volume of air it can supply and is measured in CFM, which stands for Cubic Feet per Minute. For example, if an AHU has a capacity of 4000 CFM, it means the unit can move 4000 cubic feet (ca. 113 m³) of air per minute. Accurate calculation of AHU capacity requires expertise, experience, and precision. Choosing the wrong capacity not only leads to inadequate cooling or heating but also unnecessary costs. Stay with us to learn how to calculate AHU capacity .

Key Parameters Affecting AHU Capacity Calculation AHU capacity is calculated based on the cooling and heating load of the intended space. Considering all relevant factors ensures optimal performance and comfort. These direct and indirect parameters include:

Area and dimensions of the space Geographical location Sunlight exposure on external walls Number and size of windows facing west, south, and east Presence of glass surfaces on the building facade Heat-generating appliances and lighting systems Number of occupants and their activity level Construction materials and insulation Ambient relative humidity Altitude above sea level Incoming air temperature Return air temperature Type of usage (industrial, residential, office, commercial, etc.) Air velocity across cooling and heating coils Fan pressure drop Coil pressure drop

Impact of Fins on AHU Capacity Calculation As mentioned, the material and number of fins per inch (FPI) in condenser coils significantly affect AHU design and capacity. Fins are thin metal plates designed to increase heat transfer in cooling and heating coils. They come in flat or wavy forms and are typically made of aluminum or copper, with Bluefin or Gold fin coatings for corrosive environments. FPI represents the number of fins per inch. Cooling coils usually have 12 to 14 FPI, while heating coils have 8 FPI.

AHU Fan and Its Impact on Capacity Calculation The fan is responsible for delivering the required volume of cooled or heated air to the space. Improper fan selection reduces AHU efficiency. Fan pressure drop and airflow velocity depend on the fan type used. Fan pressure drop must be accurately calculated, frequently after duct design is completed, and duct layout drawings are available.

Accurate and Optimized AHU Capacity Calculation The first step in calculating AHU capacity is to determine the cooling and heating load of the space, typically performed by HVAC engineers using software. You can contact DamaTajhiz HVAC experts for precise calculations and consultation. In general, cooling load is calculated using the formula Q = m.C.ΔT, where Q is the cooling load (Btu/min), m is the airflow rate (cfm), Cp is the specific heat of air at constant pressure (0.24 btu/lb.°F), and ΔT is the temperature difference between inlet and outlet air in °F. To calculate airflow in CFM, multiply the specific heat of air by the air density (0.075 lb /ft³) (ca. 34 g), resulting in 0.018 btu/ft³.°F. Based on software reports, the cooling load per square meter ranges from 300 to 900 Btu/hr, depending on the climate. The following table provides an estimate:

Climate Cities Residential Office Commercial Hot & Humid Bandar Abbas, Bushehr, Chabahar, Masjed Soleyman, Ahvaz, Abadan, Kish, Basra, Dubai, Doha, Kuwait, Jeddah, Muscat, Abu Dhabi, Karachi, Sharjah, Riyadh, Damascus, Mecca, Sanaa, Aden, etc. 450-550 600-700 700-900 Moderate Tehran, Karaj, Semnan, Shiraz, Mashhad, Isfahan, Gilan, Gorgan, Rasht, Ramsar, Sari, Urmia, Baku, Antalya, Beirut, Erbil, Mosul, Aleppo, Mazar-e-Sharif, Latakia, Kirkuk, etc. 300-400 350-450 500-600 Hot & Dry Yazd, Qom, Kashan, Zabol, Kerman, Zahedan, Baghdad, Medina, Karbala, Najaf, Islamabad, etc. 350-450 450-550 550-700 If the cooling load is estimated at 45,000 Btu/hr, the required CFM can be calculated as follows:

Therefore, an AHU with 4000 CFM would be suitable. However, capacity is also influenced by airflow velocity, coil pressure drop, fan pressure drop, altitude, and the type and number of condenser coil fins, all of which are considered in software-based calculations.

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Final Note

As explained, AHU capacity calculation is a complex process involving many factors, and it must be done carefully. This article aimed to provide an approximate guide to AHU capacity estimation, and we hope it was informative and practical. To explore and compare various DamaTajhiz Air Handlers or those from other brands capacity, and to check updated prices, visit the international website of DamaTajhiz (the first and most reputable company in HVAC systems since 2004, with 3 valid business licenses). For accurate capacity calculation and further guidance, call 021-88822550 during business hours to consult with our HVAC experts.

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