Less Heat Transfer When Pumping More Hot Water Through A Coil???
Less Heat Transfer When Pumping More Hot Water Through A Coil???
(OP)
Back in September, I asked a question if over-pumping a coil could ultimately result in less heat transfer. See this link to view the old thread.
http://www.eng-tips.com/viewthread.cfm?qid=351624
It was told to me by a old-timer pipefitter who said that if you over-pump a coil, the water won't have as much delta-T and therefore you won't get as much heat transfer. I of course thought I knew better because this guy was forgetting that more flow at a smaller delta-T would still give you heat transfer.
I was pretty convinced with the replies to that old thread until I read an article in PME Magazine. See attached. The article branches out to a couple of different topics, but one of them is about not getting heat transfer because of the flow being too large. I highlighted the statement that confuses me the most. I contact the author to see if he had any scientific backup for this, but he didn't. It just seems to be a phenomenon out there that you run across through experience (like my pipefitter).
Does anyone have any experience with this?
http://www.eng-tips.com/viewthread.cfm?qid=351624
It was told to me by a old-timer pipefitter who said that if you over-pump a coil, the water won't have as much delta-T and therefore you won't get as much heat transfer. I of course thought I knew better because this guy was forgetting that more flow at a smaller delta-T would still give you heat transfer.
I was pretty convinced with the replies to that old thread until I read an article in PME Magazine. See attached. The article branches out to a couple of different topics, but one of them is about not getting heat transfer because of the flow being too large. I highlighted the statement that confuses me the most. I contact the author to see if he had any scientific backup for this, but he didn't. It just seems to be a phenomenon out there that you run across through experience (like my pipefitter).
Does anyone have any experience with this?





RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
In essence this is not a thermodynamics issue, but a system water flow issue. As the flow reduces due to higher DP across the balancing valves or the bypass is closed, more flow and with a higher DP can go through the heater(s) which people want to use. The key is to look at the DT across the boiler. If this starts going down despite more heat being required, the issue is water balancing of the flow, not a new thermodynamic law(!).
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
As LittleInch stated it sounds like a circulation problem. The valves are not for temperature control on the older systems they are for pressure balancing between the different zones by equalizing the line resistances. By fully opening all of the valves the water took the easiest path, hence the shortest, and the zones furthest away got colder.
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
Simple when you think about it, but that's why the good systems should be regulating the max flow into each sub system or HX to prevent this sort of thing happening by persons who don't understand the issues of complex water networks...
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
Consider that the air flow has a limited heat extraction capacity, i.e.,
Pair = h * Ahx * dThx in watts.
The water stream has a certain level of mass transport,
vel * Apipe * ρ, which confers a power when multiplied by σ * dTwater, so the power flow in the water is expressed as
Pwater = vel * Apipe * ρ * σ * dTwater
Pwater > Pair typically, since the exchanger efficiency is always less than unity. Note that Pwater can be arbitrarily large, if vel is made arbitrarily large, yet Pair is limited to a maximum value where its dThx is essentially unchanged throughout the exchanger. Therefore, there is indeed a range of velocities where the water is increasingly cooled, but beyond a certain velocity, the amount of cooling is saturated, and the apparent effect is cooling efficiency is decreased. That is only partly correct. The cooling efficiency, as measured by h, is unchanged, it's just that you're dumping way more heat than the exchanger can handle.
Consider a simple example, say, Pwater = 1 kW, Pair = 500 W. Now increase vel by 3x, so Pwater = 3 kW, but Pair is probably only going to be about 750 W. So, in the first case, you've removed 50% of the heat from the water, but in the second case, the heat removed is only 25%, which means that the water temperature in the second case only drops by 50% of the temperature drop in the first case. So, it looks like the efficiency decreased, but the actual heat removed was increased, so technically, the efficiency went up in the exchanger, but there was more heat brought in than could be removed.
TTFN

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RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
*In the still pool scenario, a wire is immersed in a fluid and heated with an electric current, starting with the wire at or near the boiling point of the liquid. As the current is increased, (read flow), the fluid around the wire heats and begins to boil, starting with nucleate boiling which increases the heat transfer, but then transitions to film boiling quickly as nucleate boiling is in an unsteady state. This causes the temperature in the wire to heat rapidly, but reduces the amount of heat transferred to the surrounding wire, typically until the thermal overloads trip, since film boiling stabilizes around 1000 F above the boiling point of the liquid. I have seen this demonstrated, and once the current (read flow) is stopped, the system comes back quickly to the quasi-stable side of the boiling curve (nucleate boiling), which allows the current to be restarted.
Matt
Quality, quantity, cost. Pick two.
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
"Air curtains" on doors reduce heat transfer through a door by stopping a strong convective flow (cold air moving one way at the bottom of the doorway and warm air moving the other way at the top).
http://www.airtecnics.com/en/Technology/36/air-cur...
These effects are not significant in real heat exchangers and discussing them is generally a distraction from solving the actual problem. Many people get too easily distracted.
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
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RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
TTFN

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RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???
"Loiter time" has nothing to do with it whatsoever. Heat transfer is a RATE phenomenon. The molecules are always moving, vibrating, transferring their vibrational (heat) energy to the molecules of their container. Don't confuse a transient state operation with a continuous one! When people talk about adiabatic processes as processes where there isn't sufficient time for heat transfer to occur, which makes sense when considering a transient state operation, it confuses people a bit I think. In a continuous flow operation, that's a rubbish concept: what they really mean is that there is insufficient AREA for significant heat transfer to occur through. The heat IS being transferred- just not enough of it to affect the temperature of the flowing medium significantly.
RE: Less Heat Transfer When Pumping More Hot Water Through A Coil???