Boiler primary pump question

[Waramanga]

Hi all,
how common and/or advisable is it to run a boiler without a primary pump? I have come across a set-up where the 500kw boiler (700L internal water capacity) runs with zone circulators off a common header. is this a good idea? pros/cons?
cheers

 

[willard3]

If the thermal mass of the boiler is high relative to the load, it shouldn't cause problems.

Ask the same question of the boiler mfg. as different boiler materials will cause different problems.

 

[DrRTU]

First obtain minimum flow rate and minimum leaving water temperature from the boiler manufacture. Second, looking in to the control logic of the zone pumps and or zone valves. You need to run thru all conditions of startup and heating modes. Each condition must keep the boiler satisfied in terms of min flow and LWT. I have seen zone pumps used with 3way valves at the coils – this combination results in CV boiler flow.

 

[Waramanga]

thanks guys. It sounds as though it is technically ok, i mean water flow is water flow right. you are right, I should ask the manufacturer, they will most likely have recommendations on minimum flow rates, pump recomendations etc.

 

[DrRTU]

Min and Max water temperatures are just a important as minimum flow. Take a look at the Tekmar site, they have numerous controllers that have been designed to protect the systems. Its sound like you have a system as on page 7/8

http://www.tekmarcontrols.com/literature/acrobat/a260.pdf

The essay section has good design guides

http://www.tekmarcontrols.com/literature/e.html

 

[Waramanga]

thanks DrRTU, that last system is exactly like it. The system does have three way valves at least on the fan coil units. I wonder about the relative sizes of those pumps though, say for instance if one was 15% of max flow, is this ok? I will contact the manufacturer on that one too.

 

[ChasBean1]

No. It’s a bad idea. I’ve seen this just about everywhere lately with a terrible success rate. Secondary pumps have the dual job of trying to keep boilers (or chillers) happy with flow rates while doing the simultaneous job of maintaining DP at a remote secondary location. One fights the other and neither is happy about the situation.

An energy “upgrade” lately removed 40 HP primary chiller pumps at a stadium, replaced the pumps with valves, so the secondary 150 HP pumps could serve the dual purpose of primary pump plus remote DP gratifier. The larger pumps work at a much higher rate for the same duty, negating the primary pump removal savings. Ludicrous. Some compressors on the chillers fried because of imperfect controls (is that a reach?); no sympathy from the manufacturer, who said that if you remove the primary pumps you’re on your own.

Whoever is selling the “secondary pump can do it all” philosophy, is selling it very well, but they need to just go away.

The primary-secondary decoupled loop is a good invention and should be stuck with until something better comes along. Removing primary pumping to replace with control valves is not that solution.

 

[Waramanga]

Hey ChasBean, how much work is a primary pump doing if it is close proximity to the boiler, 40hp to feed 150hp secondarys, how many secondarys and is this a typical ratio between primary and secondary? In the situation you describe the 'secondary/primary' pumps are close to the boiler??
ff topic:
ESD is great for engineers to play around with technology just for the hell of it, maybe a thousand monkeys playing can come up with something useful. I get the feeling though that it is getting out of control. We are being requested to install economy cycles in small cool climate schools! I think it is the word economy that does it!

 

[ChasBean1]

I don’t have operating data before/after unfortunately. My rant was just based more on a difficult history with these single loop systems. Three adjectives to summarize thoughts on each: Primary/secondary: repeatable, simple, robust. Single loop: fluky, touchy controls, house of cards. Trips due to ‘flukiness’ make people change DP and min flow set points higher to avoid nuisances, which can negate the claimed energy savings of these setups. I advocate keeping it simple and robust. Find energy savings by means other than taking out primary pumps.

 

[EastCoastRob]

"how common and/or advisable is it to run a boiler without a primary pump? I have come across a set-up where the 500kw boiler (700L internal water capacity) runs with zone circulators off a common header. is this a good idea?"

There's no easy answer to this, and the best answer is: it depends.

- What are these zone circulators serving? Do they all serve similar loads with similar schedules? Are the zone circulators of a similar size, or do they vary?

- What is the control strategy on the boiler? On-off? Hi-lo? Modulating? Is there thermal protection on the return side of the boiler? What is the pressure loss through the boiler?

I am rather new to the HVAC game, but I've had it drilled into my skull to never starve a boiler/chiller of flow. Allowing a boiler to fire w/ only 15% of design flow through it rings many alarm bells in my head.

 

[DrRTU]

Chasbean, I was not endorsing the non-primary design just offering ideas on how to get the design to run without trashing the boiler. I believe a primary / secondary design offers the simplest and most reliable system but at the expense of additional piping and slight bump on pumping cost. In today litigious society, we need to design systems that are very robust in terms of simplicity and ease of maintenance. You will end up court faster with nonperformance than a few extra KWh. I am designing and building a twin air cooled 120 ton chiller plant that uses primary variable speed pumping. If the dp sensor(s), VFD(s) or bypass controller screw up the system will have major problems. All of this is for a data center, I am very concerned about reliability of the control system. If the design had been a primary / secondary, I would feel a little more relaxed about the stability of the system.

 

[Waramanga]

Hey DrRTU, how will you deal with the reliability issue?? mechanical failsafe ie, send pumps to max flow with mechanical pressure by-pass?? Where could one find (books web etc) a good description of chilled water circuit deign control, i am very new to this stuff and very interested!!

 

[ChasBean1]

DrRTU,

No worries, I saw your post more as presenting the facts, much better for this forum than the one-way opinion I posted. We seem to be on the same page. With the system that you describe (that seems you reluctantly “signed on” to) — I hope you note all the trials and tribulations you go through with this system, dig up this old thread, and let us know how it worked out.

 

[DrRTU]

I am a firm believer in not inventing the wheel on equipment. B&G has an excellent VFD unit that helps with the lessons learned on variable pumping.

http://www.bellgossett.com/productPages/Parts-Technologic-5500-Variable-Primary-Pump.asp

B&G has training on how the VFD works with the built in pump curve in the drive.

 

[DrRTU]

B&G has a web site with training videos of primary /secondary piping and variable piping.

http://mediasite.itt.com/mediasite/...atalogId=2fb588df-2aba-427f-8d33-66a47b396dec

 

[wyoengr]

Low flow thru a boiler = short cycle issues. Adequate flow thru a boiler + low flow thru the system, still = short cycle issues. Short cycling = poor efficiency, shorten life span.

From your brief description I would keep the boiler pump, install a buffer tank. My two-cents.

 

[ChrisConley]

What's the buffer tank for? Reducing short cyling is my guess, but I've never used a buffer tank, and don't have cycling problems.

Just to chime in, I use primary/variable speed secondary piping loops for just about every boiler/chiller loop I do.

Robust, simple and cost and energy effective.

 

[Waramanga]

ChrisConley, please divulge your technical sources to get us all on the path to low energy, high reliability nirvana! books, manufacturers data, controllers etc.....

 

[DrRTU]

This is a good paper on variable primary chiller pumping.

http://www.trane.com/commercial/library/vol31_4/adm_apn005_en.pdf

 

[ChrisConley]

low energy, high reliability nirvana!

I'm pretty sure I just said robust, simple, cost and energy effective.

Robust: good flow through the chiller or boiler. Easy to setup and a long history of stable operation.

Simple: differential pressure sensors on secondary loops are pretty standard, control sequences for variable primarly loops are not standard.

Cost effective: A small primary pump has little impact to project budget.

Energy effective: I've read articles stating that variable speed primary only pumps have the potential to save some energy over primary/variable secondary systems. I've also read articles describing numerous problems in application.