Static Pressure Help

[jaybish]

I'm trying to select the correct piece of equipment for a special project.
I have an AT&T mobile computer trailer with 10t of load on it. There are (2) 14" supply connections and (2) 14" return connections.I'm trying to obtain the static pressure of the system based on the ROT of 400cfm per ton= 4000cfm. Using a standard ductulator 14" round @ 2000cfm would= .32 friction and velocity of around 1900. WHAT WOULD MY DUCT STATIC PRESSURE EQUAL ?

 

[STYMIEDPIPER]

What is the special equipment??
Wall mount A/C unit or Floor mount ??
Your static pressure may be considerable if your equipment is at a remote location and you need to route interconnecting ductwork

 

[jaybish]

Thats my problem, whatever is used would be and could only be connected via the 14" connection hoses. The unit would be floor mount extertior to the trailor. This is a completed sealed trailor execpt for the (2) supplies and (2) returns I dont have the option of increasing duct or installing new runs. I concerned with the static as well as the coil blow bye or excessive coil velocity causing condesate to blow down the duct.

 

[STYMIEDPIPER]

You need to determine at least three (3) items to get you moving in the right direction;
1. Distance between your trailer and your remote equipment. If unknown or varies, select worst case scenerio.
2. Material of hose--PVC,metallic, nylon, etc
3. Type of hose--smooth I.D, ribbed helix, etc

Hose supplier should be able to tell you the friction loss (usually per 100 ft)of the hose that you will utilize

 

[LSA]

Let me take a stab at answering this.

The ductulator provides friction loss per 100ft. At 2000CFM in a 14"DIA duct the friction is 0.32" per 100ft of duct.

If your AC equipment is 100ft away and therefore you have 100FT of duct, your duct pressure loss is 0.32"

A more simplified approach is SP = X * 0.32"wg/100FT where X is the length of ductwork(in Ft).

 

[stanlsimon]

Be wary of pressure drop through flexible duct connections, the pressure drop can vary way higher than straight duct due to uneven inside surface, tight bending radius, kinks,
etc.

 

[zekeman]

Whatever happened to the 4000cfm you need? Why the 2000cfm?

 

[DrRTU]

Calculate the total static drop of the system. Start at one end of the system and develop a table of the loss verses flow. Your problem will not only be the flexible duct but the custom trailer’s distribution. I would start at the trailer inlet grill and find the loss for the flow, next the plenum, next the square to round and on and on. I would use tables from an industry guide such as SMACNA duct design guide. The guide has forms and examples.

http://www.smacna-oc.org/bookstore.asp

HVAC Systems - Duct Design
fundamental duct system design manual for commercial and light industrial heating, ventilating and air conditioning systems. System effects for inlets and outlets of fans, noise control in ducts and use of extractors, splinters and dampers are included. New loss coefficients for elbows with 45-degree throats on square and radius heels and radius throats with square heels. Losses compare favorably with elbows with double wall turning vanes in several cases. Structured to offer the designer options in energy-efficient design methods, materials and construction. Includes soft metrics.
348 pages, 3rd Ed., 1990
List Price: $177.00
Product: 1208

 

[crazylegs2]

As noted above a sketch is needed to calculate (appoximate) pressure drop. A 14" round at 2000 cfm would be about 1850 fpm which equates to a vp of .21. .21vp would be multiplied by each fitting C value as listed in the SMACNA design book listed above to calculate the pressure drop at each fitting. If there are no Bastards (fittings which are unusual) and you do not have any improper installation practices causing a system effect you may be able to approximate the pressure drop. In all likelyhood the friction loss of 14" round metal duct of .36" per 100 ft will be minimal compared to some fittings losses you will encounter due to the type of fittings you will be required to use to complete installation. In addition the .36" is for metal duct, flexible duct would probably be about 3x that at best estimates, and that is when it is pulled tight with no bends.In any event the SMACNA manual will shed light as to what fittings you should avoid and help you design the best possible soultion. If you will indeed be running almost all flexible then keep it as tight as possible. I suppose in the end you will probably need to purchase the system with the highest available ESP. that will fit in your application.