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# High static system

Member Posts: 2,343
You should really consult the manufacturer. They have there own calculations. Yes the blower will produce the 1.5"that you talked about, BUT with the hi rpm motor and the very small duct that is required, the trunk line static will be considerably greater. You cannot use say,....16x8 duct. SpacePak will void any warranty is their guidelines are not followed. That goes for Unico also.

Mike T.

• Member Posts: 13
HIGH STATIC SYSTEMS

WOULD SPACEPAK BE CONSIDERED A STATIC REGAIN SYSTEM?

CAN I USE A TRANE DUCTULATOR TO FIND PRESSURE DROPS IN A SPACEPAK SYSTEM?

THE ONLY INFO I COULD FIND ON THE FAN IS THAT IT OPERATES AT 1.1-1.5" OF STATIC. I KNOW THE TONAGE REQUIRED CFM AND VELOCITIES. WHAT ELSE DO I NEED TO KNOW TO START FIGURING OUT WHAT DUCT SIZES SHOULD BE OR WHATS HAPPENING WHEN DUCTS ARE CERTIAN SIZES?
• Member Posts: 25
Static Regain

is a function of the ductwork, not the air handling unit, but in this case, Hi-V systems make use of the principle.

There are two principal methods of sizing comfort conditioning ductwork- Equal friction and Static Regain.

Most designers and engineers I know use equal friction. It works and takes less time to design. It assumes that each section of ductwork incurs friction loss at about the same rate. It can result in a less expensive system depending on the number of fittings and transitions, for the duct size drops as air drops off. (Of course the cost of transition fittings has to be weighed in and that is another subject.)

Static Regain is similar but different.

First understand that aside from dynamic losses at fittings and friction losses in straight duct, there is always a measureable amount of total pressure, (Pt). Pt is the sum of velocity pressure (Pv; or "the wind in your face") and static pressure (Ps; what fills a balloon yet does not move. It makes it move but is residual to the process if that makes sense.)

Together they make total pressure. (Ps + Pv = Pt)

Think of a balloon full of air. It is full of static pressure but no velocity pressure. It is just waiting for something to do, to escape to lower pressure. You pinch the neck of the balloon, holding it in. When you release it, velocity pressure is released into the neck while the static pressure decreases inside the balloon. Total pressure still exists, but it is changing form.

This is the principle used in sizing ductwork for static regain. When air drops off at a branch, the main duct size continues at the same size for a longer distance. Loss of velocity pressure means in increase in static pressure, hence static regain.

When the velocity and hence velocity pressure drops due to the air reduction, the static pressure increases until the total pressure is met. (Total pressure does not change much, but velocity pressure changes by the square of the drop. The difference is static pressure. Such a deal.)

The net effect is, it essentially re-fills your balloon a little bit. In a SpacePak or Unico system, with consistently sized trunks, the ductwork indeed is a static regain design. One size for the most part and teensy branches coming off. By the end of the run, the static pressure is still fairly respectable, needed to get air into that 2-inch duct and out the nozzle.

Now, that balloon? Rub the balloon on a wool carpet and hold it over the thinning hair on your head. What is that called?

Static Rogaine.

Sorry- just came out.

• Member Posts: 2,343

Very nice explanation. I stood up and took note on that one:-) I am so glad you are here to put things into layman's terms. Thanks.

Mike T.
• Member Posts: 1,380

I didn't have the patience to write a novel explaining the process, so I'm very glad you did.

Thanks buddy
• Member Posts: 813

What happened to the velocity method? Carrier sells a Static Regain Duct Calculator, kind of like a Trane Ductulator only specifically for static regain. bob
bob
• Member Posts: 25
Velocity Method

Generally speaking, Bob, I do not know anyone who does the velocity method these days (by which I think you mean constant velocity). Is that what you mean?

I am familiar with the Carrier static regain calculator, never used it in design although I played around with it. United-McGill had a similar one I have used just to compare. All told, the end result had static regain requiring a design external static pressure maybe 10-15% less than the equal friction method.

But most of the systems I design are VAV so the end result is, the ductwork gets what airflow it needs. Not worth my time to calculate it out. If the static regain occurs (and it does) I benefit from it by default.

In a constant volume system, regain is also self-evident if initial balancing is performed properly. By this I mean that the furthest outlet airflow is met with the fan at the lowest possible output before any closer dampers are throttled. "Regain Happens"; you just have to set up your system to take advantage.

Constant Velocity: I have run into several systems designed and built in the 1960's using this method.

The most memorable was the NH Health and Welfare Building in Concord, NH, for which I designed an HVAC renovation in 1987-88. Building had a Multi-Zone system with finger ducts extending the length of each wing. The return was a large grille back at the shaft, open to each open-plan floor. (The return air was drawn back via the open space in the supply shaft.)

What happens in a Constant Velocity system is that the friction rate per 100 feet increases as the ducts become smaller. Those smaller ducts are smaller than if designed for constant friction.

The result? The ends of the runs were starved for air (without major throttling of upstream outlets and noise generation). Add to this, the outlets with the greatest airflow were back near the shaft. Return air had first dibs on the air supplied.

I solved the system's woes with a VAV system intercepting all and re-building the supply distribution to an equal friction method of sizing. It works now.

Otherwise, I use constant velocity for conveying systems such as dust collection or grease ducts for kitchens. Hope I grasped your concerns, Bob.

Cheers!