Esceeding chiller flow specs

[Fiziksgeek]

Originally posted by carloskoi
simple law of physics. you can only push so much water through a given pipe size. backpressure = wasted electircity which will shorten the life of your pump and waste your $$$ on electricity.

i bet the engineers who designed the chiller have actually done the calculations to tell you the max and min size pump to use so you get good efficiency without wasting your money on an oversize pump.

But if you could increase flow to make the chiller more efficient, I would rather run a pump that draws 2 amps longer, and reduce the time the 13amp chiller was on. (of course there is a alays a break even point).

My argument is that you are not increasing the overall efficiency of the chiller by increasing the flow. So not only are you using a pump that uses extra power, your chiller will probably be on longer than is really necessary.

[Fiziksgeek]

Originally posted by wlagarde
Again - wrong - I sugest you do this experiment:

Not wrong...done similar experiments, not on aquariums, but on lasers. Same situation, got heat and want to get rid of it...

Again, if it was as simple as increasing flow, why wouldnt the manufacturer tell you that?? It would be a huge selling point!

[wlagarde]

yes wrong - Actually performed experiments with a chiller in a laboratory.

[futrtrubl]

Originally posted by Fiziksgeek
Let's assume the coil is an infinite heat sink, meaning it always maintains its temperature (cold). When a molecule of water comes in contact with the coil (titanium I assume), it begins to transfer heat. It can only give up heat until it is at te same temp as the coil itself. Lets say this water molecule is in contact for 1 second, and give up 2 degrees of temperature. We push the water twice as fast. So contact time drops to 1/2 of a second, you migt only give up 1.4 degrees of temperature. So, in order to get that full 2 degree of temperature drop, the water has to go around 2 or 3 or 4 times. Dont forget, the delta T does have a significant effect, so the closer you get to the coil temperature, the smaller the delta T is, and the heat you extract. So, the second time around, at high flow, will not produce the same temperature drop that it did the first time.

Not exactly. Remember that the average deltaT for the pass is based on the average T of the molecule, roughly the average of the entry and exit Ts of the molecule. For slow flows this will cause the average deltaT to be smaller. Also, and more imprtantly, yes your molecule will have half the contact time (if doing double the flow) but you will have DOUBLE the molecules flowing passed per unit time, which cancles the halved contact time. This with the increased deltaT create more efficiency with greater flow.
This is ignoring any heat effects of the pump, which I stated in a previous post.

Edward

[wlagarde]

"Not exactly. Remember that the average deltaT for the pass is based on the average T of the molecule, roughly the average of the entry and exit Ts of the molecule. For slow flows this will cause the average deltaT to be smaller. Also, and more imprtantly, yes your molecule will have half the contact time (if doing double the flow) but you will have DOUBLE the molecules flowing passed per unit time, which cancels the halved contact time. This with the increased deltaT create more efficiency with greater flow.
This is ignoring any heat effects of the pump, which I stated in a previous post"

This is an excellent explanation - Very nicely stated.

[PrangeWay]

The first law of therodynamics indicates that the sum of the inital mass flow * initial enthalpy is equal to the sum of the exiting mass flow * exiting enthalpy. When broken down it looks as though as you increase mass flow, you will increase cooling to infinity.

BUT. As you increase the mass flow of the water, the mass flow of the coolant is constant, meaning the exiting enthalpy of the coolant is increasing. The refrigeration cycle in a chiller than lowers this coolant enthalpy back to it's initial value (ideally). There is a point, where the refrigeration cycle of chiller (with it's fixed input work, ie electricty) can no longer lower the enthalpy to it's initial value. So the h(i) and h(e) keep climbing the unit becomes less and less efficent, removing less and less heat, till it reaches a point where it is no longer functioning, the compessor is just constantly running for kicks.

The previous arguments main miss was in only looking at the heat exchanger part of the chiller.


PW

[RichConley]

Originally posted by Lo0seR
This thread is funny, I used whatever pump was available at the time for my 1/4hp chiller which was a mag18 last week. It got to 85 water temp in my prop tank in the garage (200+gal), it took forever to bring the water temp down, ya I know 1/4hp is a little to small for that tank but it works. Here is the kicker, went to the LFS store the next day got a quietone 4000 because the mag18 was just to much flow and blowing stuff all over and the specs said 7GPM anyways for the chiller. Switched out the pumps, flow is now up to specs, first thing I noticed is chiller hardly comes on now, not blow stuff around and water temp stays a nice 79 to 80.
So I think low flow is the way to go.

The mag18 pulls roughly 140w. The Quietone pulls 50w. Someone else can do that math, but I'd bet that the extra 100w of constant heating going on in your tank had more of an effect than the flow.


Prangeway, the situation you suggest is only really valid in a situation where the heat load is greater than the capacity of the chiller.

[PrangeWay]

Prangeway, the situation you suggest is only really valid in a situation where the heat load is greater than the capacity of the chiller.

Exactly! Because the coolants mass flow is fixed, you can only increase the enthalpy. The 'Heat Load' is what the refrigeration cycle can reduce from the Joules of Heat that has been removed from the water. By increasing the water's mass flow, you increase the exit enthalpy of the coolant (aka it's pulling more Joules out of the water) hence increasing the heat load. So there is an upper limit to the mass flow of water where any futher increases result in degrading the performance of the chiller.

Uh to put it even simpler, the more gph through a chiller, the more cooling you get, the more cooling the coolant needs, the more heat load you've put on the chiller, and there will be a max. A chiller doesn't just magically get rid of unlimited heat.

Theoretically this upper limit, of mass flow of the water ,before you degrade performance is what the manufacturer would list as recommended flow.


PW

[wlagarde]

We are talking about the ability of the chiller to remove heat. Not the heat a larger pump adds if it is submersed in the water. If an external pump is used as a return and the chiller is plumbed in line, it adds no heat to the water AND the skimmer still removes the same amount of heat in spite of it receiving excess flow...however, it will will run at highr back pressure. Do the experiment I suggested.

[RichConley]

Originally posted by wlagarde
We are talking about the ability of the chiller to remove heat. Not the heat a larger pump adds if it is submersed in the water. If an external pump is used as a return and the chiller is plumbed in line, it adds no heat to the water AND the skimmer still removes the same amount of heat in spite of it receiving excess flow...however, it will will run at highr back pressure. Do the experiment I suggested.

My point about the pump was that his experience could be easier explained by the change in pump heat than the chiller flow. (nothign to do with theoretical discussion)

Carry on.

[RichConley]

Prangway, it seems to me (and this is in no way my expertise) that your argument is based totally on the tank being a constant temperature source.

it seems to me that if the chiller pulls heat faster wiht a faster pump, then the tank temp would drop faster, and the chiller would shut off. It should, theoretically, need to run much less often with a faster flow.

[wlagarde]

gotcha...

[PrangeWay]

Prangway, it seems to me (and this is in no way my expertise) that your argument is based totally on the tank being a constant temperature source.

This is true. Without the coolant type, flow rate & initial coolant temperature, I can't do the calculations to see if a xxx gallon tank cools xx degrees before the chiller reaches it's limit. However it is still imporant to realize that any gain is not free, thermodynamics is all about equilibrium, any quicker cooling gained by increasing your flow will be offset by higher coolant temperaure, increased compressor work, and reduced efficency.

If assumptions have to be made, I'd say the bigger the system, the more likely it is you should stick within the recommended flow of a chiller. I personally would stay in the recommended ranges if only becuase it was designated by a mechanical engineer who knows far more about thermodynamics than I do, and about the most efficent operating parameters of the unit he designed.

[rufio173]

Ah ha, so those manufacturer numbers aren't just made up.

Thanks PW for giving another perspective on the issue.

In the end it seems like, if you're already trying to chill a system with a heat load to high for your chiller to begin with, that you should try to maintain flow in the suggested range, but if you've got a massive chiller that has way more cooling power than you need for your system, that you are pretty free to just have at it in terms of flow (within reason that is...)

I think that will help out a lot of people who read this thread and think that if I keep increasing the flow, then the chiller (no matter how small) will work for me to infinitum. Hehe. Obviously that is not the case as you have pointed out with your eloquent dissertation on the thermodynamics of our universe. Makes a lot of sense to me.

Peace,
John H.

[rufio173]

wlagarde,

Are you a resident or attending at Chapel Hill? I'm thinking of going there for my residency eventually. Hehe

Peace,
John H.

[wlagarde]

Actually faculty - but did residency and fellowship here. It is a great place (as is Charlottesville) - what are you specializing in?

Also, I'm not suggesting increasing a chillers flow wil compensate for an undersized chiller. However, increasing flow to an appropriate sized chiller will not compromise its ability to cool.

[rufio173]

I'm really debating between anesthiology with possible fellowship in critical care or thinking about urology. I like working with my hands a fair bit so I'm still debating over it although I've got to decide soon since I've just started my 4th year.

What did you specialize in?

[wlagarde]

Gotcha - UNC has excellent medicine, anesthesia, and urology programs. My only adice would be to apply and interview broadly so you get perspective in programs...

[rufio173]

Thanks, I will do that.