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

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: 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 : conditions: : -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 : min : -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 : 10-30% : -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?