esp 1

[zekeman]

I have a problem with this ESP. It is the external static pressure, external to what? If the mfr of the furnace wants you to exclude the pressure drop across the airhandler, and I picture a simplified flow system that has a series of algebraic static drops:
1) across the blower,a-b (actually a rise)
2) across the heating element, b-c
3) across the cooling coil, if any, c-d
4) from cooling coil exit thru supply ductwork to heating space,d-e
4) from heating space thru intake ducting to filter placed just before blower intake plenum, e-f
5) across the filter (assume filter before blower intake plenum) , f-a

Now, if you ignore the heating element and the filter drops is ESP the difference, c-a? If the filter is excluded then is it c-f? Or what do you think? I,m confused. And why do you think the mfr specs the esp vs max cfm?

 

[AbbyNormal]

External to the unit. read the fine print, look for if it is based on a wet/dry coil, whether the filters are included.

 

[LSA]

ESP should include items outside the unit. Items such as ductwork, elbows, transitions, filters outise unit, silencers, dampers, etc.

 

[imok2]

If you take a fan with say a forward curved blade and have nothing attached to it, the static will be 0"WG and the fan would probably overload the motor, but if you attach some duct work, filters, cooling coils,etc. you will have a resistance called static pressure or resistance to the flow of the fluid. So the mfg. determines that his fan will deliver say, 1200 cfm with a resistance of 0.5"WG so external static pressure is any resistance to the flow of the fan without any attachments. (External)

 

[ChrisConley]

If you hound the manufacturers they will give you pressure drops through their units at various airflows. If you hound them further they'll give you charts showing pressure drops across the various components in their equipment.

Once you have all that you can add your calculated external static pressure to the equipment internal static losses and specify TSP, total static pressure.

I've found however, it is easier to spec ESP and airflow. Then the onus is on the manufacturer to ensure that their fan and motor is capable of producing your spec'ed airflow.

 

[AbbyNormal]

There is a big difference between larger commerical stuff and the small unitary stuff.

The small stuff you are stuck with the limited static the equipment can handle say 1/2" or so. In this case you are forced to work with what the equipment can do.

Small packaged and split system stuff say 25 tons and under, you can take it as far as specking an over sized blower motor, high static drive options etc.

Some job where you have central station air handlers, bigger equipment, you typically deal with higher velocities. In this case the manufacturer is trying to meet your spec, will come up with the wheel and drive to suit.

As a designer you should be aware of a typical total static that a fan will face and be able to ballpark the motor size, so that the electrical designer has the right wire/breakers allowed for. Guess a little high if you have too.

 

[zekeman]

So if the mfr says that the esp includes the filter and the A coil, then my measured esp should be c-a. Correct?

 

[AbbyNormal]

While you have tried to describe the various points a, b, c etc a pitcture is worth 1000 words.

Is this an air handler with a cooling coil and electric heating elements or is is a fuel fired furnace with a blow through a coil on top?

If it is an air handler, then I would say your ESP is measured from where the supply and return ducts connect to it then.

 

[zekeman]

Fuel fired upflow with a coil on top and the coil and filter are part of the esp; so according to your nomencalure then measuring at the location between the heat exchanger and coil at one point and the input plenum at the other point. Correct?

 

[AbbyNormal]

Let's back up.

Usually a residential sized furnace will have a fan performance table givening CFM/RPM at various ESP. There will be fine print saying some thing to the effect that the tables are based 'on the filters on the filters that ship with the unit'.

So this sounds like a forced air residential furnace, that an A-coil is sitting on top of. I then mistook your comment that the A-Coil and filters are incldued in the ESP as meaning they are 'already accounted for in the CFM - ESP data provided by the manufacturer.'

Typically only packaged heat cool equipment would have a coil included already and this is not the case.

So your ESP would be taken down stream of the filter and upstream of the A-coil.

A lot of times the furnace will be rated for 1/2" ESP, and that 1/2 would then have to cover the pressure drop through the coil, ductwork, and in your case then the filter.

 

[lilliput1]

Engineers schedule external static pressure to make sure equipment vendors know what the external ductwork system would require from their equipment. They have to add their equipment specific internal losses to size the fan. Engineers got to make sure though that the designed power to the system is adequate for all vendors. So the vendor that could meet the requirement at the lowest price usually gets selected.