Leak Detection and Air Change
Per Hour Testing Services in Qatar
AIR CHANGE RATE MEASUREMENT
The provision of an adequate supply of outdoor air suitable for the needs of the occupants is an important aspect of building design and construction. Ventilation (the transport of air into, through and out of a building) can be promoted by natural or artificial forces. It is necessary to understand this process since it affects both the energy consumption and internal environment of a building. Excessive ventilation will put an undue burden on the building’s heating system and may lead to energy wastage, or an unacceptable thermal climate within the building. Insufficient ventilation can cause problems relating to the quality of the air within the building. The internal environment can become uncomfortable or, in extreme cases, harmful to the building occupants. There are a variety of methods by which buildings can be ventilated.
AIR CHANGE RATE MEASUREMENT (ACPH)
Air changes per hour, abbreviated ACPH or ACH, or air change rate is the number of times that the total air volume in a room or space is completely removed and replaced in an hour. If the air in the space is either uniform or perfectly mixed, air changes per hour is a measure of how many times the air within a defined space is replaced each hour. Perfectly mixed air refers to a theoretical condition where supply air is instantly and uniformly mixed with the air already present in a space, so that conditions such as age of air and concentration of pollutants are spatially uniform.
WHY AIR CHANGE RATE MEASUREMENT?
- The internal environment of a building must satisfy the physiological needs of the occupants. Fundamentally this implies that the occupants should be able to breath in a normal and comfortable manner.
- Respiration requires the in take of oxygen. Human tolerance to variations in the percentage of oxygen in air is relatively high and it is unlikely that a harmful shortage of oxygen will ever occur in occupied buildings.
- Removal or reduction of the source of the pollutant and dilution of pollutant once it is airborne. Since exhaled air contains carbon dioxide, outdoor air must be supplied to occupied spaces in order to dilute the internal concentration to an acceptable level. This air flow rate represents a basic ventilation requirement for occupied buildings.
METHODS OF MEASUREMENT
ACPH MEASUREMENT USING TRACER GAS
Basically, three appropriate methods exist for the determination of ACR using tracer gases.
CONSTANT INJECTION
METHOD
CONCENTRATION DECAY
METHOD
CONSTANT CONCENTRATION
METHOD
CONSTANT INJECTION METHOD
- To determine ACR with this method a diffusion tube containing Helium is frequently used as tracer gas source.
- A defined amount of tracer gas is constantly emitted over a certain period of time.
- Thus, the tracer gas concentration increases with time and reaches a stable value (equilibrium concentration) which depends on the room volume VR, the air change rate λ, and the emission rate E.
- At that time point, one or more air samples are taken and the tracer gas concentration is determined for each sample.
Concentration decay method
Tracer gas is injected into the room for a short period of time, either from a gas bottle with pressure reducer or manually from filled gas tanks.
- After mixing with the room air the tracer gas concentration is measured at regular time intervals. Because the decay curve of the tracer gas concentration C follows an exponential course when completely mixed with the room air each sampling will not only remove old air but also a certain amount of fresh air supply as well. This essentially means that there will be still ≈37% (100/e1) of the originally added tracer gas (37% old room air), after a complete air change cycle has occurred.
The variation of tracer gas concentration with time is given by
Q/V= N = Air change rate per unit time
C(o) = Concentration of tracer gas at time, t=0
C(t)= Concentration of tracer gas at time,
t = Time in minutes
CONSTANT CONCENTRATION METHOD
- During constant and thorough mixing with the indoor air, tracer gas is released in the room until a predefined concentration is reached.
- During the entire measurement the tracer gas concentration is kept constant with an automated dosing and control system.
- Under the condition of constant tracer gas concentration, the air supply is proportional to the tracer gas supply rate.
- The air supply rate can then be calculated from the ratio of the tracer gas supply to the tracer gas concentration.
- If the room volume is known, the ACR can be calculated from this ratio.
- An advantage of the constant concentration method is that even short-term changes of air supply can be detected.
- Compared to the previously described methods the technical equipment required for this method is, however, rather expensive and thus this method is comparatively rarely used for indoor air quality evaluation.
Schematics of ACPH Test Procedure
