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water heater / chiller pumping arrangement
- Thread starter ad77
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stanlsimon
Mechanical
It has to do with the air separator, you know that thing that is supposed to remove the air bubbles from the piping.
You want your warmest fluid temperature and lowest pressure to be located at the air separator. This is where the air bubbles are most likely to separate from the water.
In the case of heating, the water passes through the heater, then through the air separator, then goes to the pump.
In cooling, the warm return water passes through the separator, then to the pump, then to the chiller.
You want your warmest fluid temperature and lowest pressure to be located at the air separator. This is where the air bubbles are most likely to separate from the water.
In the case of heating, the water passes through the heater, then through the air separator, then goes to the pump.
In cooling, the warm return water passes through the separator, then to the pump, then to the chiller.
Another point to be considered is the pressure that will be exerted on the tubes .For eg assume the plantoom is in the basement of a high rise building of say 100 m high,and the pump head is 40m.
If the pumps are at the inlet,the pressure on the tubes will be 140m.If they are at the outlet,the pressure will be only 100m.This is more critical in the case of boilers because of the high teperature and therfore lower pressure ratings.
Chiller tubes generally have higher pressure rating than boiler tubes(due to the need to withstand the refrigerant pressure).
If the pumps are at the inlet,the pressure on the tubes will be 140m.If they are at the outlet,the pressure will be only 100m.This is more critical in the case of boilers because of the high teperature and therfore lower pressure ratings.
Chiller tubes generally have higher pressure rating than boiler tubes(due to the need to withstand the refrigerant pressure).
Pressure is the point, but for slightly different reasons than stated. If you are referring to a water heater, then you have a storage tank. If the storage tank is not at the highest pump head of the system, then you've wasted your pump energy by pumping into the tank - not into the system. The tank itself dissipates the pressure from the pump - that's more or less the theoretical definition of storage - a point of low pressure that collects flow and diffuses pressure heads. In that sense, the tank becomes your pumping source, not the destination.
Digression:
The only time this is not really the case is with water towers, where the elevation is used to position a constant pressure head on a hydraulic system. The advantage over straight-through pumping is that the tower has loads of buffering capability to absorb transient pressure and flow spikes, a pump does not.
In the case of the chiller, it represents a pressure-using device, e.g. - it has a significant pressure drop. If a pump is placed on the outlet, the pressure drop may exceed the NPSH rating of the pump and cause cavitation. A good design may limit these effects, but the strategy would still be contrary to the direction of goodness. Another point is that after spending $hundreds of thousands to cool the water, why would you turn around and add pump heat to the outlet?
Digression:
The only time this is not really the case is with water towers, where the elevation is used to position a constant pressure head on a hydraulic system. The advantage over straight-through pumping is that the tower has loads of buffering capability to absorb transient pressure and flow spikes, a pump does not.
In the case of the chiller, it represents a pressure-using device, e.g. - it has a significant pressure drop. If a pump is placed on the outlet, the pressure drop may exceed the NPSH rating of the pump and cause cavitation. A good design may limit these effects, but the strategy would still be contrary to the direction of goodness. Another point is that after spending $hundreds of thousands to cool the water, why would you turn around and add pump heat to the outlet?
If it's a hot water heating loop for space heating, or a chilled water loop for cooling, then the air separator location matters. Where it's located with respect to the circ pump and the heating/cooling source makes a difference. Some systems are piped incorrectly, and still work OK. But if it gets piped correctly from the get-go, it will always work properly. It doesn't cost any more to pipe it right, so why do otherwise?
???
TBP,
Perhaps I've misinterpreted, but no one was talking about air separator location, anyway. Stanlsimon interjected that as a reason for pump location relative to a water heater vs. a chiller.
(Refer to original question for this thread.)
I merely stated that air separators have nothing to do with that issue, and they don't. Systems perform quite well without air separators at all. It is far from conclusive that air separators are even needed. That has nothing to do with the correct placement of a pump in a hydraulic circuit relative to a water heater vs. a chiller.
Again, I apologize if I've misinterpreted your statement.
TBP,
Perhaps I've misinterpreted, but no one was talking about air separator location, anyway. Stanlsimon interjected that as a reason for pump location relative to a water heater vs. a chiller.
(Refer to original question for this thread.)
I merely stated that air separators have nothing to do with that issue, and they don't. Systems perform quite well without air separators at all. It is far from conclusive that air separators are even needed. That has nothing to do with the correct placement of a pump in a hydraulic circuit relative to a water heater vs. a chiller.
Again, I apologize if I've misinterpreted your statement.
The sequence of the components matters for closed loop systems, as per stanlsimon's post. The closer the water is to the boiling point (lowest pressure & highest temperature) is where the air will tend to pop out of solution. That's where you want the air separator. The location of the expansion tank is a big factor in this, as well.
It's all aimed at removing the dissolved air from the water. If you don't deal with the air issue for heating systems, it can collect in heat emitters, and you'll get no-heat complaints. The other main problem is that your system can sound like it full of BBs with the air bubbles zinging around with the water flow, and you'll have noise complaints.
It's all aimed at removing the dissolved air from the water. If you don't deal with the air issue for heating systems, it can collect in heat emitters, and you'll get no-heat complaints. The other main problem is that your system can sound like it full of BBs with the air bubbles zinging around with the water flow, and you'll have noise complaints.
I meant to include the component sequence for a heating loop:
boiler or HX, then the expansion tank & air separator (the air sep is in-line, and the expansion tank is usually connected to it with a short section of pipe), then the pump, pumping away from the separator & x-tank. Hydraulically, it doesn't particularly matter, but this sequence really does work best for dissolved air removal. The other bit, is the make-up water line. This should be fed to the short section of pipe connecting the x-tank to the separator.
boiler or HX, then the expansion tank & air separator (the air sep is in-line, and the expansion tank is usually connected to it with a short section of pipe), then the pump, pumping away from the separator & x-tank. Hydraulically, it doesn't particularly matter, but this sequence really does work best for dissolved air removal. The other bit, is the make-up water line. This should be fed to the short section of pipe connecting the x-tank to the separator.
TBP,
* sigh * No one disagreed that positioning is important. Please let me state it again, more clearly:
A. The question was how to position the pump relative to a water chiller in chilled water circuits vs. positioning the pump in water heater circuits.
B. Stanlsimon stated "It has to do with the air separator."
C. I stated it does not, and then went on to clarify the reasons for the different positions of a pump with a chiller vs. a water heater. I never stated that those positions were not critical - only, that an air separator has nothing to do with it.
An air separator is not a fundamental requirement in hydronic circuits. The position of a pump in a circuit can be critical, and is dependent on the devices it's intended to serve.
I hope that's cleared things up. If not, I give up.
* sigh * No one disagreed that positioning is important. Please let me state it again, more clearly:
A. The question was how to position the pump relative to a water chiller in chilled water circuits vs. positioning the pump in water heater circuits.
B. Stanlsimon stated "It has to do with the air separator."
C. I stated it does not, and then went on to clarify the reasons for the different positions of a pump with a chiller vs. a water heater. I never stated that those positions were not critical - only, that an air separator has nothing to do with it.
An air separator is not a fundamental requirement in hydronic circuits. The position of a pump in a circuit can be critical, and is dependent on the devices it's intended to serve.
I hope that's cleared things up. If not, I give up.
P.S. I admit that an air separator is probably a good idea for a hot water circuit, perhaps even mandatory.
It is not needed at all for chilled water.
Here's the original question:
Again, any discussion of an air separator is just "clouding" the issue (sorry for the pun). It is not essential for the hydraulics. It's only nice for the quality and efficiency. It does not dictate the position of the pump - the hydraulics of the circuit does that.
It is not needed at all for chilled water.
Here's the original question:
Again, any discussion of an air separator is just "clouding" the issue (sorry for the pun). It is not essential for the hydraulics. It's only nice for the quality and efficiency. It does not dictate the position of the pump - the hydraulics of the circuit does that.
- Thread starter
- #12
Thanks for the input.
In Australia I have not seen an air seperator on either a CHW or HHW system, so the debate has been interesting but not the answering my question.
There is no storage tank associated with the water heater. (In Australia we don't call them boilers. We only call them boiler if they produce steam. It has to do with fire rating requirements of boiler as opposed to water heaters. Another topic altogether).
We have an installation which we are currently undertaking in which the design engineer has shown the pump discharging into the water heater. We have suggested that the position of the pump should be altered and supported this with the arguement of heat exchanger pressure rating - as touched on in a couple of the posts. We don't have air seperators on this system.
The manufacturer of the water heater has indicated that are not concerened with the location of the pump on either side.
I was hoping for an engineering based arguement other than 'that's the way we always do it'.
In Australia I have not seen an air seperator on either a CHW or HHW system, so the debate has been interesting but not the answering my question.
There is no storage tank associated with the water heater. (In Australia we don't call them boilers. We only call them boiler if they produce steam. It has to do with fire rating requirements of boiler as opposed to water heaters. Another topic altogether).
We have an installation which we are currently undertaking in which the design engineer has shown the pump discharging into the water heater. We have suggested that the position of the pump should be altered and supported this with the arguement of heat exchanger pressure rating - as touched on in a couple of the posts. We don't have air seperators on this system.
The manufacturer of the water heater has indicated that are not concerened with the location of the pump on either side.
I was hoping for an engineering based arguement other than 'that's the way we always do it'.
ChrisConley
Mechanical
Just as a quick point, my main criteria for locating a pump is such that I am 'pumping' away from the expansion tank. Usually the expansion tank is located at the same point as the air seperator (but not always). The idea is that the expansion tank is the 'point of zero pressure' in the system. If I pump towards my expansion tank then all of the piping in the system becomes negative which can draw air into the system and cause other problems. If I pump away from the tank the pipe in the system is positive.
Because the air seperator is located at the warmest, lowest pressure area, and the expansion tank is often located at the air seperator I end up pumping away from boilers and into chillers.
Quick question, why don't you use air seperators in Australia? How do you remove air from the system?
Because the air seperator is located at the warmest, lowest pressure area, and the expansion tank is often located at the air seperator I end up pumping away from boilers and into chillers.
Quick question, why don't you use air seperators in Australia? How do you remove air from the system?
Let me confuse the issue further by recommending that the boiler/heat exchanger/water heater be on the positive side of the pump to increase the boiling point of water by adding the pressure of the pump.
Lots less steam flashing, especially with heat exchangers.
Lots less steam flashing, especially with heat exchangers.
ad77,
I do not know why you toss it to your heater(boiler?) vendor to make a call on the pump position.He should be advising you only on the maximum pressure rating for the tubes.It falls on you to calculate the maximum system pressure and then decide whether it is safe to locate the pump before/after the boiler.
The point in favor of having the pump before the boiler is that it is exposed to lower teperatures and may prolong its life.I do agree that the air speration point is weak to argue the case having seen installations working well without it.It might be worthwhile askig the designer the reason for putting it ahead of the boiler.
I have also seen some really high rise buildings where chilled pumps were also positioned after the chillers.
I do not know why you toss it to your heater(boiler?) vendor to make a call on the pump position.He should be advising you only on the maximum pressure rating for the tubes.It falls on you to calculate the maximum system pressure and then decide whether it is safe to locate the pump before/after the boiler.
The point in favor of having the pump before the boiler is that it is exposed to lower teperatures and may prolong its life.I do agree that the air speration point is weak to argue the case having seen installations working well without it.It might be worthwhile askig the designer the reason for putting it ahead of the boiler.
I have also seen some really high rise buildings where chilled pumps were also positioned after the chillers.
I agree with SAC9 - I have seen more chill water plants where the pump is located after the evaporator.
For hot water systems and chilled water systems that I have operated, the pump was located at the coldest point in the system.
The reasoning has been NPSH - you want the coldest fluid at the pump suction.
High rise buildings sometimes change the advantages of "pushing" through the chiller. In those cases, the return head is of sufficient pressure that NPSH is not a concern, but pressure drop through the chiller, when you already have a huge head to make up, is disadvantageous. If you are dealing with a relatively flat system, and pressure drop through the devices due to friction is the only concern, then you would most often want the pump pumping into the chiller.
The same is true if we are concerned with a boiler, not a hot "water heater" (with a storage tank). That original statement was confusing. If we are talking a hot water boiler, then you have a tube bundle in a heat exchanger, and the chiller-pump strategy in the paragraph above is just as valid - because of pressure drop.
I am glad that others recognized an air separator has nothing to do with it. If you use one, then you should be concerned about its proper placement in the hydraulic circuit, too - but that is a consequence, not a cause.
The expansion/compression tank should be viewed with the same philosphy - it is an accessory, not a prime component in the circuit.
BTW, ad77, what do you call hot water heaters (with a tank)?
The same is true if we are concerned with a boiler, not a hot "water heater" (with a storage tank). That original statement was confusing. If we are talking a hot water boiler, then you have a tube bundle in a heat exchanger, and the chiller-pump strategy in the paragraph above is just as valid - because of pressure drop.
I am glad that others recognized an air separator has nothing to do with it. If you use one, then you should be concerned about its proper placement in the hydraulic circuit, too - but that is a consequence, not a cause.
The expansion/compression tank should be viewed with the same philosphy - it is an accessory, not a prime component in the circuit.
BTW, ad77, what do you call hot water heaters (with a tank)?
The expansion tank is important in that it prevents the relief valve from lifting every time the system heats up. If there's no tank (or it's water-logged), then the relief burps out some water every time the system comes on, and the make-up water feeder adds water to the system when it cools off. This on-going addition of make-up water is what scales up boilers, results in the corrosion due to dissolved oxygen , and - wait for it - air in the system as it pops out of solution.
Having the coldest water at the pump suction is not normally a factor in closed loop heat system circulators - I've never seen one of these systems where the pressure-temperature conditions would allow the low-head circ pump to cavitate. I HAVE however, seen no end of problems from dissolved air popping out of solution at the pump suction in systems that weren't designed and/or installed correctly.
I'll see your "sigh", and raise you an "eye-roll".
Having the coldest water at the pump suction is not normally a factor in closed loop heat system circulators - I've never seen one of these systems where the pressure-temperature conditions would allow the low-head circ pump to cavitate. I HAVE however, seen no end of problems from dissolved air popping out of solution at the pump suction in systems that weren't designed and/or installed correctly.
I'll see your "sigh", and raise you an "eye-roll".
No one ever said an expansion tank wasn't needed - only that it's not the tail wagging the dog with respect to the position of the pump in the circuit. I'm sorry, but the whole point of pumping a utility medium is to serve a device - pump+device. The rest are just "nice-to-have" accessories from a theoretical basis of the point of the circuit.
We seem to be talking on two different wavelengths here, anyway, so I'm done.
We seem to be talking on two different wavelengths here, anyway, so I'm done.
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