ESP

zeke

Brad,
Thanks again. Your tutorial is excellent . I get it now.

zeke

What is ESP and how do you get CFM with this data, if at all?
If it is static pressure drop across the blower, then I understand, but I get conflicting explanations about it.

Brad White_9

External Static Pressure

is the pressure applicable beyond a given piece of applied equipment. As the designer/installer it represents the portion you have control over by your duct design, sizes of components, number of fittings, all of that.

The converse of this is ISP or internal static pressure (a term seldom used except to explain it!). This is the sum of pressure losses within a piece of equipment such as coils, dampers, casing friction, etc. You also need to compensate for filters both clean and how much they may load up before change out, the so-called "dirty filter" allowance. Generally I double the clean filter PD but this can vary.

The sum of ESP and ISP is TSP (no, not trisodium phosphate used on an internet service provider with extra-sensory perception...)

TSP or Total Static Pressure is what you will measure across the fan with the system distribution installed; that you already grasped exactly.

Hope this helps.

Brad

zeke

Brad,
Thanks so much for that coherent explanation. In that regard, if the mfr of the furnace states that the ESP includes the filter, then I take it to mean that the measured ESP is the difference between the blower intake (after the filter) and the point just past the heating element. Agree??

Brad White_116

Let me see here....

Hey Zeke-

Not exactly but let me try to delve a little deeper to cover the bases.

What you describe above, "I take it to mean that the measured ESP is the difference between the blower intake (after the filter) and the point just past the heating element. Agree??" says to me that you would be measuring the total pressure because you are measuring across the fan, only omitting the filter.

The ESP a manufacturer uses is a rating standard, what they imposed on the system (both on the discharge and on the suction side I should have stated) at the rating point when their lab was certifying the model.

In other words, say a fan/coil unit is run to deliver say 1,000 CFM at a certain fan speed and this volume is measured at a special test-quality flow measuring station in the discharge duct. The total pressure across the fan is read; say it reads 1.75 inches. Then the discharge pressure is read and the suction pressure is also read, relative to atmosphere.

In our example the discharge pressure reads +0.50 inches and the suction pressure measures -0.25 inches. Both are added together as positive integers (neat twist but true) so the ESP is thus 0.75 inches. The manufacturer then catalogs the unit as "1,000 CFM at 0.75" ESP/1.75" TSP at medium speed" in some table or other.

Filters:
When the manufacturer states that the ESP includes the filter, ideally they should define it, but are essentially saying to you, "use whatever filter you want at whatever stage of cleanliness, figure it out and include that in your total ESP over which you control." I would say that is their way of relegating the filter data to you, something over which no manufacturer can control. The same size filter will have widley varying pressure drops even when clean; furnace filters, pleated types, adheasive types, etc., so I guess that makes sense that they state that.

Brad White_116

You are most welcome, Zeke

Happy that it helped- good question and I am glad you asked it. Keeps me on my toes!