Does anybody can help on how to determine the Low pressure and High Pressure when calculate an expansion tank for heating system by using ASHRAE Fundamentals?
I'm always careful when using the term expansion tank, I assume you're refering to a compression tank, because of the reference to pressures,(closed system) and not an open tank system. The formula(s) that you use will depend on the type of tank you are selecting. An air/water or diaphram tank. If you have access to the ASHRAE Systems Handbook, both formulas are provided in chapter 12. If you don't I can provide it but it will take a couple of minutes to post let me know.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
After reading your post a second time I may have misinterpeted your question.
The high and low pressures are going to be determined by two factors. The pressures at the tank connection are selected as follows;
The low pressure at the tank connection, is selected to hold a positive pressure at the highest point in the system usually about 10 psig, when the system is cold(in the case of a heating system).
The high pressure is usually set by the maximum pressure allowable at the safety relief valve(s) without opening them, at operating temerature.
In the case of a chilled water system the reverse will be used. The high pressure will be when the system is off, and the low pressure will be when the system is at operating temperature.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
Thank you, Yorkman:
Yes, it's a closed expansion tank with air/water interface. For the high pressure, you are saying is usually set by the maximum pressure allowable at the safety relief valve without opening. Is the safe valve located on expansion tank? For our system, we have a PSV set at 150 psig on expansion tank, but there is other back pressure regulating valve set at 50 psig located on top of expansion tank as well. Which set points for our system I should use as high pressure to calculate the size of expansion tank by using ASHRAE section 12.4 equation?
I believe that if the tank is ASME rated, it must have a relief valve at the rated pressure. I think that JWengtips 150 psig valve is for tank burst protestion. In reality, your maximum operating pressure is probably much less and is usually determined by the boiler rating, which is probably the 50 psig valve. Check your boiler rating.
BronYyAur, does that mean the maxiumum operating pressure at the top experansion tank is the High Pressure which required by the ASHRAE equation.
Actually, our Boiler PSV setting point is 150 psig as well. However, the discharge pressure of circulation pump is 35 psig. Can I use this 35 psig minus friction loss and minus the head different between highest point of the system and the expansion tank?
As Yorkman said, your low pressure would be your "fill" pressure, which for a typical, non-high rise, building would be around 12 psig. Your high pressure can really be anything, as long as it's not above your relief valve setting. The higher you go, the smaller the required tank size will be. As you know from the equation, it's a function of water volume, temps, etc. Make sure you use psia and not psig in the calcualtion.
Now, you mention that you have a back pressure regulating valve set at 50 psig. I don't know why a back pressure regulator would be on a hydronic system????? But if that is your lowest relief valve setting, then you must make it your high pressure in the expansion tank calculation. Otherwise, the tank will be too small and the system pressure will reach the backpressure setting.
Regarding your pump, you are thinking of an open system. You don't have to worry about friction loss, etc. Hopefully you have the expansion tank right on the suction side of the pump? That's the best way to go in my opinion. But the bottom line is you're trying to calculate the maximum pressure. What you need to do is determine what max pressure is allowable. In reality you may never see it. So I would use 50 psig (64.7 psia) in the equation.
Yes,
BronYrAurof your right. The cold fill pressure is determined by the height of the building and making sure you have a plus 10 psig at the top of the system when full and cold. The actual setting of the pressure regulating valve (fill valve) will depend on where it is at in the system (penthouse or basement). The actual pressure value you use for the tank size calculation will be the cold fill pressure at the tank connection. This may be different than your fill valve setting.
The high pressure "setting" is going to be a function of the size you want to go with, for your tank. Like you said the smaller the tank the higher the pressure. You want to select a pressure for the tank calculation that will make the tank economical but not pop the safety valves every time the system comes up to operating temperature. Again the pressure at the tank connection will also be dependant on it's location (penthouse or basement). If the tank is located on the suction side of the pump, at the boiler (the point of no pressure change) the discharge pressure of the pump should have no effect on the tank pressure or the relief valve for that matter.
I posted a FAQ with the formulas in the HVAC FAQ section last night if they've approved it check it out.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
JWengtips, is the 50 psig backpressure valve a pressure reducing valve on the fill line. We install those on an automatic fill system, if the system pressure drops below 50 the make up water would be introduced until the system pressure was up to 50 again.
I have to explain a little bit more about our system. Our system is for industry buildings unit heating. The highest point for our system is around 10 meters. Our expansion tank is at suction of pumps and located about 3.3 meters high. We have compress air (pressure 100 psi) injects to the expansion tank through a regulation valve (set @ 45 psi). There is another back pressure regulation valve (set @ 50 psig) in a vent line located at the top of expansion tank. ASME rated valve (ste @ 150 psi) at the top of tank as well.
To me, it sounds as though the system is designed to keep a minimum pressure of 50 psi and maximum of 150 psi. This would ensure the 45 psi required by the process while maintaining overpressure protection for the tanks rating.
I'm with insult on this, are you running medium temp water in the system above 122[°]C (250[°]F)?
Your static pressure is only about 15 psig plus the 10 psi for positve pressure at the top, so I'd say cold fill disregarding what your process needs, would be 25 psig excuse the cross up on metric. If your settings are going to be 45 psig ok BUT, where does this, "There is another back pressure regulation valve (set @ 50 psig) in a vent line located at the top of expansion tank" vent to?
If it's set to vent at 50 psig to atmosphere, then your operating range is going to be low pressure (cold)45psig; and high pressure(operating temperature)50psig. It's an awful tight pressure range and will probably require a fairly large tank to keep with in that parameter.
I might need some more data as I'm not sure I've got this cold pressure you have correct, what is the standing cold pressure of the system? Let's say at the pump everything off boiler cold pumps dead
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
The system max.operating temperature is 121C(250F). The heat medium is 50% Glycol and 50% Water.
This is an existing system. My duty is to check the size of existing expansion tank to see if it's still adquated due to the capacity of the system increased (more unit heaters added). Therefore how to decide the P1 & P2 becomes very critical. It may lead to completely different results.
The back pressure regulation valve vents to atmosphere through a Catch Tank.
The pump dead head is 35 psi.
Joan, that sounds like fun! (I've been doing some reading) I think the backpressure regulator is actually designed to vent compressed air from the tank to maintain a constant tank pressure +/- 5 psig. The 150 psig relief valve would be better located at the bottom of the tank so a liquid seal is formed to insure your relieving water and not air.The cold fill pressure sounds like it should be 45 psig, simply because your regulating system is going to try to maintain when cold. It also sounds like the backpressure is going to try to maintain a max pressure of 50 psig in the compression tank
I think the compression tank will need to be sized using those two pressures to keep from constantly venting air back to your holding tank. This could make for a rather large tank I don't know. The compression tank should also have a high and low water level alarms to let some one know there is a malfuction of the pressurization/water make-up system. The ASHRAE HVAC Systems and Equipment Handbook Chapter 14.3-14.6 Fig. 8-9-10 has a pretty close discription to this type of system. Just so you know that I'm not making this up. I hope this is of some help the material did not really go in to tank sizing in that chapter, but chapter 12 in the same Handbook does.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
Joan,
If your client doesn't need the 250[°]+ water for a process any longer. Would converting the system to a standard low temperature below 250[°]system, be a possibilty? Converting the compression tank to a typical air/water tank would be easy lose the compressed air system and backpressure valve. The bigger question would be main sizing and existing water flow. At lower water temperatures the mains may be undersized and the water circulation rates to low. Just some food for thought.
I'm not a real engineer, but I play one on T.V.
A.J. Gest, York Int./JCI
