Uniform definition of ESP

[CptHk]

What is the definition of External static pressure?

I was asked this because of a problem my PM encountered and I am having a hard time finding the actual definition in ASHRAE. I find the term everywhere and I thought it was easy to know and understand, but I can't find the definition.

I found thread403-164474 to be useful but did not get my final answer I was looking for.

The problem site has a AHU (CRAC specifically) with draw through fan with supply duct, diffusers, return grilles and return duct. All other particulars should be irrelevant to the conversation.

Is the ESP measured on supply side only or does it include all supply side losses and return duct right up to the unit?

I assumed it would include all external losses, but cannot find confirmation anywhere.

 

[TADiep]

You are correct. ESP is the measure of static pressure losses from ductwork, fittings, dampers, grilles and any other devices located in the airstream.



----
A green thought..."We don't inherit the earth from our ancestors, we borrow it from our children." (unknown)

 

[ChrisConley]

Not to be confused with TSP, which is total static pressure, ESP is all external losses that the fan must work against.

If you don't have a return fan, then your supply fan must work against the return duct, return grille, etc.

 

[willard3]

Static pressure is, in the first place, only part of the problem.

Total pressure is what the fan has to over come, the sum of static and velocity losses.

You will have less problems with purveyor substitutions if you calculate total pressure and use that in the specifications. Let the purveyor worry about his equipment's pressure drop and whether it will meet the operating criteria.

 

[CptHk]

Chris,

That is exactly the definition I thought it was, is it defined in any ASHRAE or TABB publications?

I scoured all the ASHRAE publications, but I did not find the definition of external static pressure. The Engineering Toolbox has a HVAC terms section but the definition of ESP is lacking in my opinion. Maybe the term is so obvious it is never explicitly defined.

 

[lilliput1]

External static pressure would be whatever pressure loss there is that is not caused by the components of the airhandler itself except it is based on the filters being clean. With variable speed fans, in addition to the external static, you should specify a dirty filter allowance that the AHU manufacturer should include in sizing the fan and motor. Otherwize you will have to accept the reduction in CFM as the filters get dirty. For constant volume units, you have no choice but to accept this drop in CFM. If you provide excess static pressure capability in CV fan, the balancer will either just throttle down dampers or reduce the fan speed. No one will readjust the dampers or fan speed when the filters get dirty.

 

[imok2]

External static pressure (ESP) is measuring the Static Pressure on negative side and the then the positive side of a fan, coil, or heat exchanger. Adding these two numbers together (disregard the fact that one is negative) gives the ESP. Use a static pressure tip for proper results. The tip pointing into the air flow)
I've seen the terms total static pressure, external static pressure, and total external static pressure used interchangeability. Look at it this way Total Static Pressure is what you calculate, EPS is what the manufacturer says it should be once the unit is connected to external ductwork.