Hot Water Boiler Efficiency

[macmet]

Hello Everyone,

I tried using the search on this website for this topic but I came with hundreds of hits. I love this website but the search isn't the best.

My question is in regards to the efficiency of a hot water boiler as a function of boiler feedwater rate. We have two hot water boilers that run at low firing rates right now, <20%. They are sized for a complete system, and we are less than 1/6 of the way there. Note, these boilers never run at the same time.

However, our boiler feedwater pumps are running at 100%, all the time. I would think that not only is that excessive pump horsepower consumption, but that it would also reduce the efficiency of the boiler because the water flow through it far exceeds the requirement.

I'm told that pump flow through the boiler has no bearing on the efficiency. This seems reasonable in typical operating conditions, but in low firing rates like this, I am not sure if it still applies.

As always, I'd appreciate any feedback.

 

[Compositepro]

I seems you are referring to a water circulation pump and not to feedwater. High circulation rate (the design rate) will improve heat transfer so it should not be a problem. Lowering the flow could lead to steam generation when you have a hot water (not steam system).

 

[macmet]

CompositePro,

I guess you are right, they should probably be called circulation pumps.

And I agree, if we were to slow the flow it is possible to create steam, but I'm thinking there is a more appropriate level than 100% flow. It just does not seem right to me to be operating the boilers at low firing rate and having the pumps push as much water through them as possible.

We have less than 20% firing (typically less than 15%), but our flow is 100% of the pump capacity, not necessarily even the design flow rate.

These are 500BHP boilers.

 

[TBP]

Why not just call the boiler manufacturer? He should be able to provide the part-load information you're after.

 

[macmet]

I've contacted the boiler manufacturer but have struggled to get information. I continue to try though and think eventually I may get somewhere.

I was hoping someone might have a curve or experience with this sort of issue.

 

[Compositepro]

Your process is a heater. Where do you think any pumping losses are going? It will show up as heat in the water. So this energy is not lost. If you use less electricity you will only have to fire the boiler more, but I doubt you could measure it.

 

[TBP]

Compositepro - It's much more than that. You're converting electrical BTUs (very expensive) to thermal BTUs (relatively MUCH cheaper).

Not all BTUs are created equal...

 

[macmet]

My concern is that the lower temperature at the exit to the boiler makes the heat exchangers down the line slightly less efficient. Not to mention the additional losses caused by the higher flow rates.

To me it just does not seem right to have the boiler firing at such a lower firing rate while the pumps are working at full flow.

I hope to hear more from the manufacturer tomorrow. But I like hearing other people's opinions that are removed from the situation. I appreciate all the comments so far and hope for more.

 

[TBP]

How low are "low temperatures" of the water leaving the boiler? If they're too low, you'll condense flue gases in the boiler, and the resulting condensate is very corrosive.

 

[macmet]

We're aware of the issues with low temperature flue gas and try to keep the temperatures up above 75C (which I still think is low).

These boilers are fairly new amd we haven't seen any corrosion issues as of yet, but I've seen some severly corroded boiler tubes over the last few years.

 

[Compositepro]

I recently did some comparisons on energy cost at a consumer level in Tennessee and there was little difference between electricity, propane, natural gas, and fuel oil. Electricity was not the highest one either. So I ask, how will slowing the flow save any significant energy?

 

[macmet]

I think slowing the flow, might save some energy, yes. But, I do not know for sure, which is why I asked. I initially thought that thermal btus were cheaper as well. Perhaps this depends on the location of the plant?

But it's not just that. It's slowing the flow may raise the boiler outlet water temperature, which may lead to increased boiler return water temperature, which may reduce the risk of condensation in boiler tubes. And after a week of investigating our system paraments, this is now my primary concern.

Also, higher boiler outlet temperatures provides higher hot water inlet temperatures to our heat exchangers downstream, which may also raise the efficiency of these heat exchangers. This is not something I know for sure yet, it is something I am investigating. I'm still new to these types of systems. But from what I recall in school, the greater the delta t, the better for heat transfer.

But, if it does improve efficiency, maybe we do not need to provide heat for as long and we can fire less, which saves fuel costs.

 

[macmet]

I just reread the last post I made and I think it sounds more confrontational than I intended.

I really do appreciate all responses and consider everything people say or suggest.

Have a good weekend,

Macmet

 

[pipesnpumps]

I designed a very large boiler system last year. The circulation pumps were designed to control a fixed supply pressure. In addition, there is a low-flow bypass valve that bypasses water from the supply header to the return header in order to maintain the minimum required flow through the boiler.. In my case it was around 40% of the design flow..

This saved some energy but not alot due to the flatness of the particular pump curve I had to use. It did pay for the variable speed drive though. The amount of energy it saves depends on the shape of the pump curve.

In my case the system taking the heat load had 2-way valves, so the flow demand varied.

In your case, it sounds like the system taking the heat load is designed to take a constant-volume. So you should not reduce the circulation flow unless the system is also re-designed for variable flow.

 

[RayWK]

Let's not forget our pump laws. Remember, 1/2 the flow takes only 1/8th of the power. So, if you can reduce the pump flow and still service your loads adequately, by all means do it!
I think Mr. pipesnpumps has the right approach regarding loads. Mr. Macmet: What kind of loads do you have (coils, rads, infloor, etc)? Are the attached loads controlled by 2-way valves or 3-way valves? A little more info could help point us all in the right direction.

 

[macmet]

Well, this system is for district energy. So we have a few heat exchangers located at various distances from the boiler plant. We can run our boilers at 14% most of the time, and meet the heat requirements of all of our heat exchangers. When the boilers are running at 14% the pumps are running at 100%. When they run at 21% our pump runs at 100%.

We do have a valve (which I assume is considered a 3 way valve) downstream of the boilers that allows us to recirculate from the supply to the HX directly to the return line to maintain the minimum pump flow.

I should also add more information. We have a distribution pump downstream of the boilers that delivers the water to the heat exchangers. Initially I thought this wasn't relevant but now I'm wondering if it is. It makes me wonder the purpose of the pumps upstream of the boiler.