In this article, we will learn how to calculate air changes per hour or ACPH or ACH along with formula, calculation, table. In air conditioning or ventilation system design, the calculation of ACPH is one of the vital criteria, hence, it is essential to have a basic knowledge of it.
Let’s explore!
What is Air Changes per Hour or ACPH or ACH?
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: Air Changes Per Hour
- ACH: Air Change Rate
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. We will learn the detailed calculation and how ACPH or ACH is considered in it.
Air Changes per Hour Basic
In ventilation design, there may be a 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 need to be removed,
- The room may require a temperature set like my room will be limited to a maximum of 3 or 5 or 6 deg. C temperature above the atmosphere, etc.
- There may be a chance of smoke inside the room which needs to be released to the atmosphere.
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 that is to be added or removed from an area to the volume of the area. If Q amount of air enters into the room of volume of V, then Air Changes per Hour = Air quantity / Volume = Q/V
- 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.
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What Does Air Changes per Hour or ACPH Mean? Meaning
Let’s try to understand the meaning of ACPH with a few examples,
What Does 1 ACPH Mean?
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,
What Does 5 ACPH Mean?
5 ACPH means,
- 5-time air enters to a room and exits from the room in 1 hour, or
- Air fills into the room 5 times in 1 hour,
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Air Changes per Hour Formula
What is the Formula of Air Changes per Hour in S.I. Unit?
Let us see, how do we calculate the air changes per hour? To calculate the ACPH in S.I unit, we have to do the following,
- Need to measure the supply or exhaust air quantity, by using any device, like a 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 m3/hr (Cubic meter per hour or CMH)
- L = Length of room, in m
- W = Width of the room, in m
- H = Height of the room, in m
- V = Volume = L x W x H, in m3
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 value is 6, and in a cable cellar, the APH value is 4. These values are predetermined and available in various standards like ASHRAE.
How Do You Calculate Air Changes per Hour with Formula in F.P.S unit?
After deriving air changes per hour, let’s see how to calculate air changes per hour in F.P.S. To calculate the ACPH, we have to know the following,
- Need to get the air quantity
- Need to get the room size (length, width & height)
- Calculate the volume,
ACPH will be air quantity divided by room volume,
Mow, consider,
- Q’ = Air quantity, in ft3/min (Cubic Feet per Minute or CFM)
- Q = Air quantity, in ft3/hr = 60 Q’
- L = Length of room, in ft
- W = Width of the room, in ft
- H = Height of the room, in ft
- V = Volume = L x W x H, in ft3
ACPH will be air quantity divided by room volume,
That means,
Air Change Rate with Different Units
Air change rate is expressed in the below equation,
Q = V x ACPH
or ACPH = Q / V [When Q & V are in S.I Unit]
or, ACPH = Q x 60 / V [When Q & V are 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 m3/hr
- V = volume of the in m3
F.P.S Units
- Q = Air quantity in C.F.M (Cubic Feet per Minute)
- V = volume of the in ft3
ACPH Explanation
If 7200 m3/hr air is supplied to 120m3 volume room,
- ACPH will be = Q / V
- ACPH = 7200/120
- ACPH = 6
Hence, ACPH =6
If 800 CFM air is supplied to 4800 ft3 volume room,
ACPH will be = Q x 60 / V = 800 x 60/4800 = 10
[800 CFM = 800 CFM x 60 ft3 /hr]
Hence, ACPH =10
Standard Air Changes per Hour or ACPH Table or Chart
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 Room | ACPH or ACH |
|---|---|
| General Areas | 0.5 – 1 |
| Corridors | 2 – 4 |
| Meeting rooms | 4 – 6 |
| Normal Stores | 6 – 10 |
| Chemical Stores | 10 – 15 |
| Educational Facilities | 4 – 6 |
| Student Classrooms | 6 – 12 |
| Laboratory | 4 – 6 |
| 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 |
How to Calculate 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 Air Quantity
If the air quantity of a particular room is known the same data can be used to calculate ACPH. If it is not available, it is required to measure the air quantity.
- Say, Measured Air Quantity is, Q in m3/hr
Step #2 Measure Room Length
If the room length of a particular room is known the same data can be used to calculate air change rate. If it is not available, it is required to measure the length of the room.
- Say, Measured Room Length is, L in m
Step #3 Measure Room Width
If the room width of a particular room is known the same data can be used to calculate air changes per hour. If it is not available, it is required to measure the width of the room.
- Say, Measured Room Width is, W in m
Step #4 Measure Room Height
If the room height of a particular room is known the same data can be used to calculate air changes per hour. If it is not available, it is required to measure the height of the room.
- Say, Measured Room Height is, H in m
Step #5 Volume Calculation
Now, in this step, room volume needs to be calculated. We have got the value of room length, width & height.
- Calculate the volume,
- V = length x width x height = L x W x H in m3/hr
Step #6 ACPH Calculation
We know air quantity (Q) and room volume V. So, from the definition of air changes per hour or ACPH, we can write:
- Air changes per hour (ACH or ACPH) = Q / V = Q / (L x W x H)
Example of ACPH or Air Change per Hour 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 = 2500 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 m3
V = 250 m3
As per formula,
ACPH = Q / V = 2500 / (10 x 5 x 5) = 10
Hence, with the simple mathematical calculation, the value of ACPH becomes 10.
Is the ACPH 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 m3
We know from the standard ACPH list,
- ACPH for electrical room = 6
- Or, Air quantity, Q1 =ACPH x 300 = 6 x 300 = 1800 m3/hr
Now, if 1800 m3/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 is 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/m3
- ∆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 m3/s
- or, Q2 = 12.0 / (1.2 x 1.005 x 5) in m3/s
- or, Q2 = 1.99 m3/s = 7177 m3/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 and we have to select whichever is higher.
ACPH or ACH 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 m3
Air quantity, Q = V x ACPH = 60 x 12 x 10 x 10 = 7200 m3/hr
Heat Load Method
Q2 = q / (Cp x ρ x ∆T)
or, Q2 = 14.4 /(1.2 x 1.005 x 5) m3/s
or, Q2 = 2.39 m3/s = 8604 m3/hr
Hence, 8604 m3/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 m3/hr (8604/3) nos.
Exhaust fans, 3 nos. 2868 x 0.8 = 2295 m3/hr.
The numbers and capacity of supply fans and exhaust fans shall be based on the building layout, and 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, and 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 for ACPH
Refer to ASHRAE 62.2-2016, “Ventilation and Acceptable Indoor Air Quality in Residential Buildings” for ventilation details.
FAQs for Air Changes per Hours
10 ACPH means, 10-time air enters to a room and exits from the room in 1 hour, or
Air fills into the room 10 times in 1 hour,
Air changes per hour in a room depends on the applications. If its a case in our room where we stay, it can be 0.5 to 1 or sometimes, 2.
If it is a battery room, then it has to be increased and normally it is considered as 6-12 depends on the type of battery.
For Bathrooms, air changes per hour is considered around 6.
To calculate Air Changes Per Hour (ACPH), first it is required to measure the air quantity and room volume.
Then simply make ratio of air quantity and room volume.
ACPH = Q / V [When Q & V are in S.I Unit]
ACPH = Q x 60 / V [When Q & V are in F.P.S Unit]
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Conclusion
So, finally, we have got the basic idea about air changes per hour or ACH or ACPH along with various examples, and diagrams.
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