How to select AHU? Explain in detail.

- Coil selection based on temperature difference for air, water, and velocities.

- Selection of filters according to specs requirement and face velocities.

- Fan selection based on internal, & external static pressure air flow rate.

- Humidifier selection depend on RH needed.

AHU Selection needs lot of considerations as follows.

1. The tonnage shall be as per requirements.

2. CFM shall be as per requirements.

3. The filtration shall be as per requirements.

4. Type shall be as desired like its with upward flow, downward flow, is it above ceiling or in separate room, is it with heater or heat recovery system, is it for fresh air only or for mix air.

5. The humidity shall be as per requirements.

6. Some other requirements like body type, casing type, fan type.

7. Fan selection from highest pressure drops in ducts and air inlet outlets.

8. Control selection like it is coming with which types of controls, is it standalone type or can be controlled by BMS only.

All above options can be further divided but these are basics. Hope this is clear

Air handling unit (AHU) is mainly with Fan, coils (heating & cooling) & filters. As per project requirement add UV lights & humidifier.   

1.      Always keep in mind that the capacity required and selected capacity should mach with the requirement.

2.      While selecting Give priority related to Performance, efficiency, maintainability, and space constraints to select a unit that has the lowest lifecycle cost for a given application.

Fan Selection

1.      Fan must generate required air pressure to overcome internal and external  static pressure

2.      More than5 hp  required to have a minimum FEG of67 and will need to operate within10% of their peak efficiency

3.      A critical part of any fan selection is acoustic performance. Proper selection and specifying of fans and AHU casing can reduce the need for silencers / sound attenuator.

(The best way to reduce fan sound is to reduce the fan power; efficient fans frequently have the best acoustic performance.)

Coil selection

Coils can be selected for Chilled Water, Hot Water, Steam, Refrigerant, and Refrigerant Heat Recovery & Coil Run-around Loop Heat Recovery

Cooling coil selection shall be selected on the base of environment and operation situation (copper tube /copper fin or copper tube with Al fins), coil frame (proper material say SS316), corrosion protection coating etc.

Filters selection

G-filters – based on average arrestance (gravimetric method) with ASHRAE52.1 test dust at final pressure drop of250 Pa

• M- Filters - based on average particle size efficiency at0.4 μm at final pressure drop of450

• F-filters – based on minimum efficiency and average particle size efficiency at0.4 μm at final pressure drop of450

 Note

Select a correct filter with lowest optimized pressure drop

This filter will create significant savings on energy

1 additional Pa over an air filter adds extra energy costs

Other accessories

UVC lights (ultraviolet light in the C band) reduce the growth of bacteria, mold, and algae on coils and drain pans.

Humidifiers: 

Depending upon the location, external fresh air flow, difference of moisture content between indoor / outdoor conditions required steam capacity will be arrived. Depending upon the required stream capacity humidifier selected. 

how to determine the load in tons then give that to someone who sells air conditioning or refrigeration units, install the components of the unit/s purchased that include the ahu, and do not forget to pray for Allah and his messenger prophet muhammad saw

you, as a design engineer should give some information to the manufacturer you want to buy the AHU. since, these information should be noticed:

1-The tonnage of your cooling/heating load calculation(I rate it by HAP4)

2-the AHU air flow(I calculated it by HAP)

3-kind of your zoning(single zone/multi zone...): depends on your system service

4-The kind of your Coils( child/hot water coils or DX cooling coil, or Electrical heating coils)]

5-one of important point is the place of your AHU's Installation that would say what kind of air-thru should your AHU to be(front-thru, upright/dnright stream) and the limitation o your AHU Dimension

* for further information, you should talk with your manufacturer,,

I agree with both answers

AHU selection is a crucial task in the execution stage. All parameter pertaining to the selection criteria shall be optimally decided & listed with recommended safety factor as per application & performance duty. Deciding the components of AHU is the major task at the initial stage of equipment selection. Depending upon the application for which it shall be employed for, the designer shall specify all the mandatory component as well as additional components. Thermal load, Air quality &energy consumption shall be the points to focus while selection. Depending upon these3 aspects & manufacturer's product range, AHU equipment shall be specified with the desired performance rating. In some cases, especially healthcare applications, tailor made products suffice the requirement when it comes to meeting stringent codes.