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Need some advice on multi-boiler design w/ prim/sec loops+inj. pumps

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Posted by jamie on March 07, 2010 at 02:24:54:

hi everyone! first post need some advice on our system.

So we have a smaller industrial system which consists of jacketed tanks and a hot water system. Max

system temp requirement on secondary loop is ~180F. Due to the propensity of jacketed tanks to dump

large volumes of cold water into the system, it is setup as a primary/secondary loop system with an taco

007 injection pump controlled by a tekmar 356 between the loops.

for the boiler side we have the following two boilers (in parallel on primary loop):
-oil fired cast iron boiler (peerless), max btu of 175K, cut down to ~150K right now via smaller nozzle.
-modulating propane-fired condensing boiler (trinity Ti200, 200k btu)

we are a biodiesel manufacturing plant so we would prefer to use the oil boiler where possible but i do

realize the propane boiler could be a lot more efficient due to its modulating and condensing design.

i dont think the condensing boiler needs the same low-temp return protection as the oil fired boiler, so id

like to arrange them to where the modulating boiler provides low-temp heat using a limited amount of

fuel and then turns it over to the oil boiler for the bulk of the load.

heres the rub:
ive been test firing it with the oil boiler only, its not working how i would like

-oilboiler@1.35 GPH with aquastat set to boiler on at 170, off at 195.
-taco 0011 on primary loop, ~10 psi pressure drop (~10 GPM flow thru primary loop)
-tekram 356 is mixing hot water via taco 007 as injection pump and controlling boiler return temp to 150F

-the simulated 'worst case scenario' load ive been testing with is secondary loop at ~10 GPM (guesstimate).

Secondary loop temp drops below 100F when i open up heating to a tank jacket.

what happens when i run this system:
-when the sec. loop temp is so low, the inj. pump tries to inject more water, but this pushes more cold

water back to the primary loop, dropping boiler return temp to 150F which limits injection pump output to

-primary loop temp rises too fast and boiler cycles frequently (5 min on/5 min off). i cant go any wider on

the aquastat @ 25F
-secondary loop is not heating as fast as id like it (should be utilizing boiler full time at this point)

ok, so i need some way of buffering temperatures to be able to move more heat from prim to sec without

dropping prim loop temp too much

so as a buffer I am thinking of:
-adding a water storage tank in the primary loop (~100 gal, 16000 btu of storage from 170-195deg.
-adding an intermediate loop with more storage
-attempting to use existing process loads as heat sinks to modulate temp (not as operator friendly, nor efficient)

any suggestions?

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