Minimum flow for air removal

rb_6

Hi Scott,

Here's a couple of presentations on the topic.

<A HREF=" http://www.healthyheating.com/Page%2055/Page_55_h_heating_eq_air_separation.htm"> Air Separation </A>

<A HREF=" http://www.healthyheating.com/Page%2055/Page_55_h_heating_eq_velocity.htm"> The Effects of Velocity on Air Separation and Heat Transfer</A>

Mark Eatherton had some interesting observations on high performance micro bubble types (Spirovent) vs what I call the boat anchors types...traditional cast purgers. I have a zippy separator on my system and occasionally it has had challenges with delivering on its promise. Would I go back to a boat anchor on my next house...good question?

We have a number of different HTU’s and the radiant wall probably operates at the lowest of all velocities, estimated at a nominal 1.1 fps at 30 deg F delta t. We have a coin air vent at the pinnacle of the wall and tubes are sloped to facilitate purging. The velocities in the fan/coils and radiant floor are much higher...a nominal 3.5 fps.

I don't have instrumentation to verify so it’s purely a theoretical calculation.

I’m a big fan of designing for larger delta t’s…there is a recommended chart in this presentation:

<A HREF=" http://www.healthyheating.com/Page%2055/Page_55_h_heating_eq_circ_flow.htm"> Calculating Flow Rates </A>

My general observations is the trick to low velocity and large delta t’s is making sure the system is going to be piped to facilitate air and sediment removal. Once this is taken care of, all things being equal, designs that are more aggressive can be considered.

RB

Scott Lind_4

Being an engineer (electrical) I tend to listen to other engineers but I'm getting conflicting opinions on the need to maintain flow rates above 2fps for air removal as recommended by John Siegenthaler and ASHRAE.

Does anyone pay attention to this or ignore it? Do the new air vent designs change this recommendation? I'd like to slow some design flow rates down to get higher deltaT numbers on radiator circuits.

Seems like in a system with TRVs the design flow is rarely going to be there anyway. Anyone experiencing air problems in systems with lower flow rates?

Empire_2

Itt Fluid handling...

According to the design/training literature water volocity in the hydronic system should be high enough to carry entrained air in the water stream,yet not so high as to cause a noise problem. Water velocity should be above 1.5 to 2.0 ft/sec. towards the point of air seperation,(vents, air seperators etc...) In pipping larger than 2" velocity's in excess of 4 ft/sec can be used. Flow rate must be maintained in the main loop to (at least) aid in the elimination of entrained air. (We all pay attention to this for optimum performance). You may also increase you Delt. "T" but be sure to decrease your line size for the total BTU/Hr and the flow rate to maintain your 1.5 to 2.0 ft/sec flow rate to aid in removal of entrained air. Any slower,..I has a tendancy not to move and eventually becomes a problem..
Hope this helped. Mike T.
Ref....Bell&Gossett.com///
Dan H could you please chime inn.....? He's good.:)

thp_8

It depends

if you want trouble free operation or if you occasionally want to play the (chasair) game. Secondly yes alot of the tube jobs you see with a 3/4" line feeding 1 or 2 loops and then we add a power purge to get the big air out NOT. Velocity one of the biggest keep secrets in hydronics or a practice just not followed. Third we like ugly air scoops.

Scott Lind_4

Thank You

for the reply Robert. I scanned your references quickly and will dig into them tonight. I appreciate the detailed information - it helps me grasp "the why".

George Peteya_2

Design for Velocity

I always design for velocity rather than delta T. For pex, I use 0.5, 0.9, 1.25, and 1.75 USGPM for 3/8", 1/2", 5/8", and 3/4" respectively. These flow rates give you a little less than 2 feet per second. At these flow rates, and the "normal" maximum loop lengths of 200', 300', 333' and 500', you can still specify low head (read: low noise)circulators too.
As an example, for a radiant zone with four 1/2" pex circuits, the design flow rate is 3.6 gpm. Then I calculate the delta T. If the floor area covered by the four loops is, say, 900 sf, and the heat load is 20 Btuh/sf or 18,000 Btuh, the delta T is 10°F. IMHO, there is no reason to break the rules of piping design, just because it's a heating system! (Remember where that 20°F. delta T rule came from: it made it REAL EASY to calculate flow rate based on heat load. All you had to do was divide the load in Btuh by 10,000 and you had the flow rate in USGPM).

Scott Lind_4

Pipe Size

Mike,

Thank you for the reply. This is a situation where the install got ahead of the understanding. I had the heating contractor running pex to all the radiators in advance of understanding the whole issue of laminar flow, air removal, etc. Putting 3/4 pex in place of the old 1-1/4" and 1-1/2" black iron seemed risky. Knowing what I do now I'd have 5/8" or 1/2" put in to keep the velocities above 2 fps and still keep delta T values higher.

Now I'm looking to make the best of an existing situation.

Scott

hr

It's not uncommon to run small

diameter pex to radiators if it is a home run system. panel radiators are often fed with 1/2" pex, sometimes 3/8"

1/2" pex running a 40° delta t, if you can design around that, can easily move 60,000 btu/ hr. at under 4 fps velocity. That could feed a pretty healthy sized radiator

hot rod

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