**Air changes per hour** or **air change rate** is a very common term in ventilation calculation as well as ventilation design. It is abbreviated, as,

- ACPH
- ACH

We need to do ventilation calculations for different areas in different industries and ACPH or ACH is one of the main factors for that calculation.

**What are Air Changes per hour?**

**Air Changes per hour basic**

In ventilation design, there may be few things,

- The room may require fresh air,
- The room may have more CO2 gas, which needs to be replaced by fresh air,
- The room may have chemical or toxic gases
- The room may have fumes, which needs to be removed,
- The room may require a temperature set like my room will be limited to a maximum of 5 or 6 deg. C temperature above the atmosphere, etc.

These all cases are associated with the ventilation system. How do we do this ventilation system?

It is simple, we add air or remove air from the area by considering air changes per hour.

**Air Changes per hour definition**

It is basically the ratio of the volume of air which is to be added or removed from an area to the volume of area.

- It is assumed that air into the area is uniform.
- Mixing of air is proper inside the area.
- Maintain uniform velocity.
- It can be stated as the number of times the air is entered into the area and exits from the area in one hour.

**ACPH meaning**

1 ACPH means,

- 1-time air enters to a room and exits from the room in 1 hour, or
- Air fills into room 1 time in 1 hour,

5 ACPH means,

- 5-time air enters to a room and exits from the room in 5 hours, or
- Air fills into the room 5 time in 1 hour,

**Air Changes per Hour Formula**

Let us see, how do we calculate the air changes per hour? To calculate the ACPH, we have to do the followings,

- Need to measure the supply or exhaust air quantity, by using any device, like thermometer or velometer or vane anemometer,
- Need to measure the room length, width & height,
- Calculate the volume of the room,

ACPH will be air quantity divided by room volume,

That means,

Where,

- Q = Air quantity, in
- L = Length of room
- W = Width of the room
- H = Height of the room
- V = Volume = L x W x H

Note, ACPH is defined in various standards and it is fixed for specific areas or specific applications. Suppose if it is a battery room, the ACPH for battery room is available in various standards to use it.

**Air Change Rate with different units**

Air change rate expressed in the below equation,

**Q = V x ACPH**

or **ACPH = Q / V** in S.I Unit

or, **ACPH = Q x 60 / V** in F.P.S Unit

Where,

ACPH = air changes per hour

Q = Air flow rate

V = volume of the space or room

**SI Units**

Q = Air quantity in m^{3}/hr

V = volume of the in m^{3}

**F.P.S Units**

Q = Air quantity in C.F.M (Cubic Feet per Minute)

V = volume of the in ft^{3}

**Explanation**

If 7200 m^{3}/hr air is supplied to 120m^{3} volume room,

ACPH will be = Q / V = 7200/120 = 6

Hence, ACPH =6

If 800 CFM air is supplied to 4800 ft^{3} volume room,

ACPH will be = Q x 60 / V = 800 x 60/4800 = 10

[800 CFM = 800 CFM x 60 ft^{3} /hr]

Hence, ACPH =10

**Standard ACPH rate**

ACH or ACPH is based on industry, national standards, international standards, client-specific requirements as well as location. However, based on our experience, a reference list is given,

Name of Area | Air Changes per Hour (ACPH) |
---|---|

General Areas | 0.5 – 1 |

Corridors | 2 – 4 |

Meeting rooms | 4 – 6 |

Normal Stores | 6 – 10 |

Chemical Stores | 10 – 15 |

Educational Facilities | |

Student Classrooms | 6 – 12 |

Laboratory | 6 – 12 |

Library | 4 – 10 |

Office Buildings | 0.5-1.5 |

Office Rooms | 0.5-1.5 |

Municipality Buildings | 0.5-1.5 |

Public Assembly Spaces | 0.5-1.5 |

Halls for public gathering | 4 – 6 |

Auditorium | 4 – 12 |

Electrical Room | 0.5 |

Pump Room | 10-15 |

Battery Room (Lead-acid) | 12-13 |

Battery Room (Ni-Cd) | 4- 6 |

**Calculate of Air Changes per Hour**

We have already learned the formulae of calculating air changes per hour, here, look at all the simple steps for the calculation in S.I units

**Step 1**

Measure the air quantity, Q in m

**Step 2**

Measure the length, L in m

**Step 3**

Measure the width, W in m

**Step 4**

Measure the height, H in m

**Step 5**

Calculate the volume,

V = length x width x height = L x W x H in m^{3}/hr

**Step 6**

Air changes per hour (ACH or ACPH) = Q / V = Q / (L x W x H)

**Example of ACPH Calculation**

Let’s take a simple example. You are in a room and the room needs to be ventilated. The size of the room is 10m x 5m x 5m.

Measure air quantity = 2000 CMH,

So, what is the air changes per hour?

Calculation

- Air quantity, Q = 2500 CMH
- Length, L = 10m
- Width, W = 5m
- Height, H = 5m

Hence, Volume V = 10 x 5 x 5 m^{3}

= 250 m^{3}

As per formulae,

ACPH = Q / V = 2500 / (10 x 5 x 5) = 10

**Is ACPH is the only method for ventilation calculation?**

In the case of ventilation calculation, ACPH is not the only method by which the calculation or the equipment selection is done.

For example,

Let us consider, an electrical room which has heat-generating equipment. To understand, consider,

- Electrical room length = 10m
- Electrical room width = 6m
- Electrical room height = 5m
- Equipment heat generation = 12.0 kW

So, volume = 10 x 6 x 5 = 300 m^{3}

We know from the standard ACPH list,

ACPH for electrical room = 6

Or, Air quantity, Q1 =ACPH x 300 = 6 x 300 = 1800 m^{3}/hr

Now, if 1800 m^{3}/hr is provided in the electrical room, the room will be ventilated, but what about the equipment heat, generated by electrical items?

Will it be restricted to be heated?

No, unless, a separate calculation method to be adopted. This second method is called the heat load method.

A standard formula for this method,

Heat, q

- q = m x Cp x Δt
- q = ρ x Q x Cp x Δt
- Q = q / (ρ x Cp x Δt)

Where,

- q = Heat Generated in kW = 12.0 kW
- Cp = Specific Heat = 1.005 kJ/kg-K or kJ/kg-Deg. C (Same)
- ρ = Density of air = 1.2 kg/m
^{3} - ∆T = temperature difference between outside temperature and inside temperature= 5ºC [∆T depends on the project requirements, here it is 5ºC]

As per the equation,

Q2= q / (Cp x ρ x ∆T) in m^{3}/s

or, Q2 = 12.0 / (1.2 x 1.005 x 5) in m^{3}/s

or, Q2 = 1.99 m^{3}/s = 7177 m^{3}/s = that is > Q1

Hence, Q2 is more than Q1.

Air quantity in the ACPH method is less than the heat load method, in this example.

Hence, during ventilation calculation, both methods are to be considered, as in many cases, it varies.

**Project Example**

Let us take a project example,

**Reference drawing**

**Input Data**

From the drawing, the following data, are as follows,

- Length of pump hosue, L = 60m
- Width of pump house, W = 12m
- Height of pump house, H = 10m
- ACPH = 10
- Heat Load data, q = 14.4 kW
- Temperature difference = 5ºC

**Design Calculation**

**ACPH Method**

Volume of pump house, V = L x W x H = 60 x 12 x 10 m^{3}

Air quantity, Q = V x ACPH = 60 x 12 x 10 x 10 = 7200 m^{3}/hr

**Heat Load Method**

Q2 = q / (Cp x ρ x ∆T)

or, Q2 = 14.4 /(1.2 x 1.005 x 5) m^{3}/s

or, Q2 = 2.39 m^{3}/s = 8604 m^{3}/hr

Hence, 8604 m^{3}/hr air quantity is selected.

**Equipment Selection**

The pump house needs to be positively ventilated, which means supply and exhaust to be provided and supply shall be more than exhaust.

As per drawing, 3 nos of supply fans, each of 2868 m^{3}/hr (8604/3) nos.

Exhaust fans, 3 nos. 2868 x 0.8 = 2295 m^{3}/hr.

The numbers and capacity of supply fans and exhaust fans shall be based on the building layout, space availability.

**Why ACPH is an important factor?**

ACPH is very important to design a ventilation system, and proper ACPH selection incurs a proper ventilation design. A proper ventilation system helps, as follows,

- It removes pollutants from the ventilated space.
- It supplies fresh air,
- Improves indoor air quality (IAQ)
- It helps to maintain inside temperature, humidity up to a certain level.
- It can help pressurize the space by supplying more air and exhausting less air.
- It gives a certain level of comfort to human beings.
- It restricts to increase the temperature up to a certain level which helps all equipment to maintain it’s limited temperature.

## Standards

Refer to ASHRAE 62.2-2016, “Ventilation and Acceptable Indoor Air Quality in Residential Buildings” for ventilation details.

## Conclusion

So, finally, we have got the basic idea about air changes per hour or ACH or ACPH along with various examples, diagrams.