What level of cab pressure is "standard."
The cab pressure demo was starting, and already I could tell it was going to get interesting. It was during the winter of 2011, when I was in the tar sands mining mecca, Ft. McMurray, Alberta, Canada, with a sales rep for a cabin pressurizer company. He was excited to share their system, which could generate extraordinary cab pressure even in a leaky cab. As I listened to the whistling sound getting louder and louder, I sympathized with the operator who would be forced to listen to this high-pitched whistle for hours at a time. The pressurizer worked so well that the cab pressure was well over 300 Pa, and the pressure against the cab door and window seals created leaks that whistled as the pressurized air forced its way out of the cab. I got into the cab, and the salesman slammed the door closed. I immediately felt that I was in an airplane, and the pressure was changing rapidly, causing my ears to beg for mercy. Understanding the implications of cab pressurization was officially on my radar screen.
There are different upper and lower threshold limits for cab pressure articulated in standards. Continuous cab pressure keeps dust from coming through leakage points into the cab. Dust that comes into the cab from a door or window or on the operator's clothing is drawn through the recirculation filter and removed from the cab.
So why was a cab pressure range of 20 Pa to 200 Pa made the performance requirement in ISO 23875:2021? The pressure sensors used to measure low pressure have a margin of error that, by the standard, is ±10 Pa. A minimum of 20 Pa ensures that air is constantly forced from the cab, not allowing dust to enter through cab leakage points.
The rationale for the upper-pressure limit is more interesting. The leakage points must be reduced to achieve high pressure in the cab. As cab pressure increases, the volume of intake air goes down. As we saw in the previous post on CO2 levels, it takes a minimum of 25 CFM of intake air to dilute the human-generated CO2 and maintain acceptable oxygen levels. The trade-off is high cab pressure for lower intake air volume.
As the intake air filter loads with dust, the amount of air coming through the filter declines until there is not enough intake air drawn into the cab to maintain a safe oxygen level. Filter life is determined by the difference between the highest cab pressure achieved when the intake filter is new and the low-pressure threshold of 20 Pa. The objective is to create enough cab pressure to ensure that the intake filter life is equal to or exceeds the replacement interval of the intake filter.
Cab pressure is a variable in the equation and not the result. The result is acceptable air quality which places CO2 mitigation/acceptable oxygen levels as the first objective, followed by controlling the dust levels in the cab. Pressure helps to accomplish these objectives, but it is not an objective in itself. The difference between the upper and lower pressure thresholds of 180 Pa is sufficient to allow the intake filter to achieve the planned maintenance interval.
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