Hot Water Tank with 2 Pumps and H.Ex.

[enenenen]

We have a tank to be used as hot water tank has a diameter of 34”. Tow pumps are connected to the tank one is 60 gpm takes the hot water from the tank to the building. The second one re-circulates within the Heat Exchanger (Steam – Water Heat exchanger, gets steam 110 psia). City water pipe 46.4 ºF is connected to the Heat exchanger.

The water level in the tank moves up to 47” highest from the tanks base, when the water level becomes at 47” city water closes, the re-circulating water pump 90 gpm with in the heat exchanger work re-circulate the water while the 60 gpm pump taking water from the tank. When the water level reaches 31” from the bottom of the tank re-circulating water stops the City water 75 gpm 46.4 ºF ºF solenoid valve open pass city water to the tank through the heat exchanger with a temperature of 106 ºF

I would like to calculate the water temperature at any point for the two situations
1- When water level in tank is in its highest level until it goes down to its lower point 31” from the bottom of the tank (How to calculate water temperature at each point until it reaches its lower point)?
2- When water level in the tank in its lower level going up (How to calculate at each point until it reached highest level which is 47” from the bottom of the tank)?
3- Water temperature for the 60 gpm at any time?
Note: as mentioned above the city water only open when water reach it lower level until the tank is full at that time re-circulating pump is not working. While re-circulating pump works when water is in its higher level going down at that time city water is closed. (The out pump 60 gpm is always ON)

One thing else I would like to add is that the Heat Exchanger water out temperature become 106 ºF only when the City water is in with a temperature of 46.4 ºF. Sure it will be deferent when re-circulating the water with the 90 gmp pump.

Thanks in advance

 

[chicopee]

Based on your muddy description of the operation, since the water temperature(only supplied from city water)come out of the H.X. at 106dF when the tank is filling, I would tend to believe that the water temp in the tank stays at 106dF until it fills up. When the tank is emptying (while the recirculating pump is operating) the temp also stays at 106 dF. I am assuming that you have no water return from the building during filling and emptying of the tank.

 

[enenenen]

No, that is not right. the tank never come empty and it does not (only supplied from city water). Maximum hieght it reaches is 47" from the bottom of the tank and minimum hieght it goes down is 31" (Not zero (empty)). there for it vary 16" between maximum and minimum. If the situation is as you mention, it would be easy.

The 106 dF water temp. our of H.Ex. is only when city water is supplied (not when re-circulating pump works).

But I mentioned when the water becoms at its maximum level the city water stops, the re-circulating pump starts until water reaches to the minimum which is 31" hieght from the bottom of the tank. then when the level reaches its minimum the city water starts, the re-circulating pumps stops contiuning until it reaches to maximum level which is 47" from the bottom of the tank and so on.

Thanks.

 

[IncogNico]

enenenen,

To be able to help you out without a lot of back and forth,

all I need are your specifications.

by the sounds of it you have a storage tank and a heat exchanger. all i need are the conditions of the heat exchanger loop and the domestic.

Hot Side:
110 Psi Steam
(approx. 344ºF)

Cold Side:
Temp In - 46.4
Temp Out - 106?
@ 75 GPM

Is this about right?

If not, provide me with the basic specifications without a lot of wording (sometimes it just complicates things.)

 

[enenenen]

Yes that is right.

For the Recirculating pump gpm = 90 work only when city woter is Off.

Thanks.

 

[IncogNico]

To be able to maintain a “relative” temperature of 106ºF as the output you must have at least 2558 lb/hr which will be a total HX duty of approx 2,250 MBTU/hr.

Once the recalculating pump kicks in, assuming the Steam flow above, the max output temperature that you could achieve would be approx 96ºF.

This is all depending on the type of exchanger that you have installed. Some designs are less efficient than others.

There are ways that this system could be more efficient, and this would be by simply adding another storage tank in the loop and draw all of the needed water form the tank. By doing this you would create, in my opinion, a more efficient system and you would be able than to increase the output temperature if needed.

Realistically there are many different ways that this system could be set up to be a little bit more efficient.

I hope this helps…

 

[enenenen]

At the beginning lets assume that the system just started Now (That means the temperature in the tank and only for the first time will be 106 Deg F):

Once the tank reaches to its maximum level 47" from the bottom, the city water stops the re-circulating pump 90 gpm kinks in (While the out put pump 60 gpm is always ON) So while the water level in the tank go down to minimum level 31" the temperature water in the tank rises above 106 Deg F continuously until the water in the tank reaches highest level (So at that time the water in the tank and as you mentioned with the max. opening of steam to H.Ex gets for sure wormer than 106 and so on now again mixing 106 with wormer in the tank.. With that for sure the out put water in the 60 gpm will always be more than 106. And that is what I would like to calculate besides the water temperature at any point when it is going up and while it goes down in the tank? Once the tank reaches to its maximum level 47" from the bottom, the city water stops the re-circulating pump 90 gpm kinks in (While the out put pump 60 gpm is always ON) So while the water level in the tank go down to minimum level 31" the temperature water in the tank rises and rises above 106 Deg F continuously until the water in the tank reaches highest level (So at that time the water in the tank and as you mentioned with the max opening of steam to H.Ex gets wormer than 106 and so on now again mixing 106 with wormer in the tank.. With that for sure the out put water in the 60 gpm will always be more than 106. And that I would like to calculate besides the water temperature at any point when it is going up and while it goes down?

 

[IncogNico]

If the system is already installed to get accurate results I would install a thermostat at the output. This way you can read the temperature through all of its stages. I am sure that you will find that it will never be constant and will always be fluctuating.

Unfortunately unless I spend a lot of time to this will not be able to calculate the varying temperatures of the output flow.

What type of heat exchanger are you using for this application?

 

[enenenen]

Yes, you are right there is a fluctuating. There were no other way at that time to do. getting hot water with out increasing the H.Ex. size. (The re-circulating pump 90 gpm was only 50 gpm before replacing it).

Regarding results rather than practical testing. Sure there is a math to calculate what I am lookin at?

Bisides do you thing there is another way to do so?

Thanks.