Air CompressorAir Discharge Volume : Technical Data
Normal
The air flow which the compressor takes in a certain time unit is called the air volume (or air quantity), and it is expressed by Q (m3/min). It is generally called the gas volume, but it is specifically called the air volume if the air is taken in. If a fan is used, it is also called the air capacity.
In terms of compressors, the air volume is not expressed by the air discharged, but always expressed by the air converted by the temperature, pressure, and humidity at the air inlet of the compressor, even if the air is used under the pressure of the discharge side. In other words, if the air volume is mentioned, it always means the intake air volume even if it is not specifically said so (JIS-B-8341, JIS-B-0142 No.2008).
Since air changes its volume a great deal depending on the pressure and temperature, to say about the air discharged at a certain place, the pressure and temperature of the place must always be stated. To avoid such trouble, the intake air volume is used.
Thus, the air is expressed converted by the intake conditions of the compressor, but in some cases, it is expressed by converting to a reference condition (0ºC, 760mmHg, dry state).
In this case, it is expressed as Nm3/min, where N stands for the N in Normal Condition.
Therefore, if the air volume is expressed as Nm3/min, it must be converted to the intake condition m3/min when selecting compressors or calculating power.
For example, take the case where the air volume expressed as 1000Nm3/min is converted to the air volume under the intake conditions whose temperature is 20ºC, atmosphere pressure is 760mmHg and humidity is 50%.
First, calculate the specific gravity of the air of 20ºC, 760mmHg and 50% humidity from the following formula.
| γ = | 0.465 × | 750-0.378 × 0.5Χ17.5 273 + 20 |
| = | 1.185kg/m3 | |
Since the normal condition is γ = 1.293, the intake air volume Q is calculated as follows.
| Q = | 1000 × | 1.293 1.185 |
= | 1090m3/min |
| = | 1.185kg/m3 | |||
Since the volume increases by approximately 10%, you must be careful not to overlook N. In this sense, the intake air volume m3/min is sometimes written as Bm3/min to alert you. Note that the unit m3/h is sometimes expressed as m3/s or by weight kg/s.
The intake air volume means the wet air of temperature 20ºC, absolute pressure 760mmHg, and humidity 65% (JIS-BO142), and the normal condition air volume means the dry air of temperature 0ºC, absolute pressure 760mmHg, and humidity 0% (JIS-BO142). The conversion from the intake air volume is calculated with the following formula using the temperature, atmosphere pressure, moisture pressure and humidity under the intake condition.
| Q s = | Q n | × | Ts 273 |
× | 1.033 Ps–Ψ × Pv |
| = | 1.185kg/m3 | ||||
Where:
| Q s : | Intake air volume | (m3/min) |
| Q n : | Normal condition air volume | (Nm3/min) |
| Ts : | Intake temperature | (°K) = (273 + tsºC) |
| Ps : | Intake pressure | (kgf/cm2abs) |
| 1.033kgf/cm2(abs) = mmHg | ||
| Ψ : | Relative humidity | (%) |
| Pv : | Moisture pressure under intake temperature | (kgf/cm2 abs) |
Calculation Example
*The permissible value of the discharge air volume is ±5% of the intake air volume (JIS-B8341).
Explanation
As shown in the calculation above, the conversion from the intake condition to the normal condition is affected by the temperature, atmosphere pressure and humidity.
If the intake air temperature is higher than the atmosphere temperature, the normal condition air volume further decreases.
If the discharge pressure is changed during operation, the air volume does not change but the horse power consumed changes.