"parallel" flow or primary/secondary - utilize more exchanger capacity
"parallel" flow or primary/secondary - utilize more exchanger capacity
(OP)
This is hard to explain without a diagram, but I'll give it a try. I am trying to add onto an existing hot water heating system and I am trying to determine which option is better. I have a heat exchanger sized for 308 GPM with ent and leaving water temps of 180 deg to 220 deg (40 deg delta). Currently, the unit is operating at only 180 deg discharge and 160 deg entering (20 deg delta). I want to utilize the excess capacity of the heat exchanger for a separate heating operation. My new requirements are 180 GPM with a 20 deg delta.
Option 1 would be to "parallel" my new pump to the existing. My new and existing pumps would both pull through the heat exchanger, but then the water flows would go their separate ways. Both returns would mix right before going back into the heat exchanger. My concern with this approach is the combined flow through the heat exchanger. All I know is that it is a shell and tube rated for 308 GPM with a 2-1/2 ft water pressure drop. I don't know how it would perform or what pressure drop to expect with a combined 488 GPM.
Option 2 would be to use the existing pump as a primary and pull off the discharge with my new pump. I would then return my new flow righ back to the supply line, thereby creating a secondary loop. This of course will drop my available supply temp from my original system, but the heat exchanger should be able to compensate. Now I am running 308 GPM of 180 deg water. If I "steal" 180 GPM and return it at 160, my new mixed supply temp in the 308 GPM line will be around 168 degrees. It would seem that all I have to do is change my heat exchanger setting to 208 deg discharge in order for my mixed supply to be back to the original 180 deg. This is still only a 32 deg delta on my heat exchanger, which is rated for 40 deg. My concern here is that my mixed supply temp will fluctuate as my new load varies. I thought of moving my heat exchanger discharge water temp sensor downstream of the mix point to compensate for this. So I will be controlling the mixed water temp and not the heat exchanger supply temp.
Any thoughts on which approach makes the most sense? Any better ideas?
Option 1 would be to "parallel" my new pump to the existing. My new and existing pumps would both pull through the heat exchanger, but then the water flows would go their separate ways. Both returns would mix right before going back into the heat exchanger. My concern with this approach is the combined flow through the heat exchanger. All I know is that it is a shell and tube rated for 308 GPM with a 2-1/2 ft water pressure drop. I don't know how it would perform or what pressure drop to expect with a combined 488 GPM.
Option 2 would be to use the existing pump as a primary and pull off the discharge with my new pump. I would then return my new flow righ back to the supply line, thereby creating a secondary loop. This of course will drop my available supply temp from my original system, but the heat exchanger should be able to compensate. Now I am running 308 GPM of 180 deg water. If I "steal" 180 GPM and return it at 160, my new mixed supply temp in the 308 GPM line will be around 168 degrees. It would seem that all I have to do is change my heat exchanger setting to 208 deg discharge in order for my mixed supply to be back to the original 180 deg. This is still only a 32 deg delta on my heat exchanger, which is rated for 40 deg. My concern here is that my mixed supply temp will fluctuate as my new load varies. I thought of moving my heat exchanger discharge water temp sensor downstream of the mix point to compensate for this. So I will be controlling the mixed water temp and not the heat exchanger supply temp.
Any thoughts on which approach makes the most sense? Any better ideas?





RE: "parallel" flow or primary/secondary - utilize more exchanger capacity
2.5 ft * 488^2 / 308^2 = 6.3 ft
I assume this is ºF and you're not planning on boiling anything, but if pressures are not increased somewhat, low pressure somewhere in the system may get below the new higher vapor pressure at 208ºF causing the water to vaporize. Will you still have NPSHR at the pumps with the new temps? Water might now also vaporize at <control> valves, or perhaps during a surge event.
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: "parallel" flow or primary/secondary - utilize more exchanger capacity
Thanks. So would you recommend option 1?
RE: "parallel" flow or primary/secondary - utilize more exchanger capacity
RE: "parallel" flow or primary/secondary - utilize more exchanger capacity
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com