How do you calculate the size of AHU?

How do you calculate the size of AHU? Calculating the size of an Air Handling Unit (AHU) involves several steps and considerations to ensure that the unit can adequately meet the heating, cooling, ventilation, and air distribution needs of the space it serves. Here’s a detailed guide on how to size an AHU:

 Determine the Cooling Load

1. Calculate the Total Cooling Load (Q):
   • Internal Heat Gains: Includes heat generated by occupants, lighting, and equipment.
   • External Heat Gains: Includes heat from solar radiation, conduction through walls, windows, and roofs.
   • Ventilation Requirements: Fresh air that needs to be conditioned.

   Use the following formula:

   Q=Qinternal+Qexternal+QventilationQ = Q_{\text{internal}} + Q_{\text{external}} + Q_{\text{ventilation}}Q=Qinternal​+Qexternal​+Qventilation​

2. Cooling Load Formula:$$Q=1.08\times CFM\times \Delta T$$Where:
   • $Q$ is the cooling load in BTU/hr.
   • $CFM$ is the airflow in cubic feet per minute.
   • $\Delta T$ is the temperature difference between the return air and supply air.

 Determine the Heating Load

1. Calculate the Total Heating Load:
   • Internal Heat Losses: Consider heat loss from the building envelope.
   • External Heat Losses: Includes conduction through walls, windows, and roofs.
2. Heating Load Formula:$$Q=1.08\times CFM\times \Delta T$$Where:
   • $Q$ is the heating load in BTU/hr.
   • $CFM$ is the airflow in cubic feet per minute.
   • $\Delta T$ is the temperature difference between the return air and supply air.

 Calculate Airflow Requirements (CFM)

1. Determine Ventilation Requirements:
   • Based on ASHRAE standards or local building codes, calculate the required ventilation air (outside air) for the space.
2. Calculate the Required CFM:
   • For cooling: CFMcooling=Qcooling1.08×ΔT\text{CFM}_{\text{cooling}} = \frac{Q_{\text{cooling}}}{1.08 \times \Delta T}CFMcooling​=1.08×ΔTQcooling​​
   • For heating: CFMheating=Qheating1.08×ΔT\text{CFM}_{\text{heating}} = \frac{Q_{\text{heating}}}{1.08 \times \Delta T}CFMheating​=1.08×ΔTQheating​​

 Determine Supply Air Volume

1. Supply Air Volume (CFM):
   • The larger value between the cooling and heating airflows will typically determine the required supply air volume.
   • Ensure the CFM meets both heating and cooling requirements, considering the greater demand.

 Calculate Fan Sizing

1. Determine Total Static Pressure:
   • Calculate the total static pressure in the system considering ductwork, filters, coils, and other components.
2. Select Fan:
   • Choose a fan that can deliver the required CFM at the calculated total static pressure.

 Select Coils and Filters

1. Cooling Coil:
   • Select a cooling coil that can handle the cooling load at the required airflow and pressure drop.
   • Consider entering and leaving air conditions to ensure the coil meets the load requirements.
2. Heating Coil:
   • Select a heating coil that can handle the heating load at the required airflow and pressure drop.
   • Ensure the coil meets the load requirements with the entering and leaving air conditions.
3. Filters:
   • Select filters with appropriate MERV ratings that can handle the airflow and minimize pressure drop.

 Ensure Adequate Ventilation

1. Outdoor Air Intake:
   • Ensure the AHU can introduce the required amount of outdoor air for ventilation.
   • Verify that outdoor air dampers, economizers, and controls are sized to handle the ventilation requirements.

 Confirm Energy Efficiency and Compliance

1. Energy Recovery:
   • Consider incorporating energy recovery ventilators (ERV) or heat recovery ventilators (HRV) to improve efficiency.
2. Compliance:
   • Ensure the AHU design complies with relevant codes and standards (e.g., ASHRAE, local building codes).

Example Calculation

Given:

• Cooling Load: 60,000 BTU/hr
• Heating Load: 40,000 BTU/hr
• Desired Supply Air Temperature Difference (Cooling): 20°F
• Desired Supply Air Temperature Difference (Heating): 30°F

Calculate Cooling Airflow (CFM):

CFMcooling=60,000 BTU/hr1.08×20 °F=2,778 CFM\text{CFM}_{\text{cooling}} = \frac{60,000 \, \text{BTU/hr}}{1.08 \times 20 \, \text{°F}} = 2,778 \, \text{CFM}CFMcooling​=1.08×20°F60,000BTU/hr​=2,778CFM

Calculate Heating Airflow (CFM):

CFMheating=40,000 BTU/hr1.08×30 °F=1,235 CFM\text{CFM}_{\text{heating}} = \frac{40,000 \, \text{BTU/hr}}{1.08 \times 30 \, \text{°F}} = 1,235 \, \text{CFM}CFMheating​=1.08×30°F40,000BTU/hr​=1,235CFM

Select Supply Air Volume:

• The larger value between cooling and heating is 2,778 CFM.

Conclusion

• The AHU should be sized to provide at least 2,778 CFM of airflow.
• The fan should be selected to deliver 2,778 CFM at the calculated static pressure.
• Coils and filters should be chosen to handle this airflow while meeting the cooling and heating loads.
• Ensure the design includes proper ventilation, energy efficiency measures, and complies with relevant standards.

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