The flow-equivalent values of OA-CHANGES and OA-FLOW/PER, multipliedįACTOR. Otherwise the reported value is the maximum of If OUTSIDE-AIR-FLOW is specified, its value is multiplied by the ALTITUDEįACTOR and reported here.
Reflects the user-specified outside air quantity entered at the zone level. The ZONE keywordsĪIR-CHANGES/HR and FLOW/AREA will be accepted by SYSTEMS only if they areĬonsistent with the user-supplied HEATING-CAPACITY and COOLING-CAPACITY andĪre equivalent to a flow larger than that of the exhaust from or the Will the value here correspond to your input. Have specified a value for the ASSIGNED-FLOW keyword in the ZONE command Is the calculated or user-specified supply flow for each zone. Note: the quantities in this report (SV-A) have been adjusted forĪltitudeeven though DOE-2 requires that any flows you enter in SYSTEMS The excerpts below discuss the areas of eQUEST impacted by the altitude In real life, however, the designĮngineer is calculating and specifying supply and outside air cfm that isĪlready adjusted for the altitude and the altitude factor should simply be The write upīelow suggests that one calculate both supply and outside air at sea levelĪnd then apply the altitude factor to it. If you are working with some type of fixed, off-the-shelf designs, then you should have some really significant safety factors built into the selection.The altitude factor effects the supply air and the outside air. Normally, you would select your fan based on the actual ACFM and static for the given conditions. However, I think you are going about this in a somewhat backwards fashion, unless you are moving an existing unit to a higher altitude and want to see if it will still work. Here's a quick altitude density correction factor from a well-known fan manufacturer: So as as the altitude becomes higher, the acfm stays constant, the mass flow decreases, and the static decreases. Generally, pressure drop calculations are on the order of mass velocity squared divided by density. In that case, to answer your last question, yes, the static pressure across the cooler bundle changes with altitude. I understand that you are designing some type of air-cooled exchanger, right? Reidh RE: fan perfomances at high elevation imok2 (Mechanical) 25 Feb 10 20:35 We could open up another can of worms here if this cooling question was related to a naturally aspirated engine (less power produced at altitude), but we will leave that for another day. It is clear that the decrease in air density easily offsets the decreased temperature, resulting in reduced heat rejection as altitude increases. 7% temperature decrease per 1000 ft near SL, compared to an approximate 3.5% decrease in air density per 1000 ft. Since heat transfer is proportional to mass flow rate, a higher pressure altitude will have a negative effect on a system's heat rejection capability due to the lower air density.Ĭounteracting the negative effect of lower air density with increased altitude is the fact that temperatures decrease at a rate of 2 C/1000 ft up to 36,000 ft (assuming a standard lapse rate). To answer your original question, the volume flow rate for a fan should be independent of altitude.įans are typically used to aid in heat transfer.
Given a few assumptions, a fan will move a given volume of air per revolution regardless of air density or temperature. Fans (or propellers) are constant volume devices.
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