Propane and condensate

MikeG

Is there a theoretical amount of condensate that would be generated from burning propane in a mod con?  I have Munchkin M80 with mostly copper finned BB and a small section of radiant, also an indirect DHW.  Just trying to get a feel for what would be possible. I know efficiency, supply and return water temp etc ect play a role.  I have been measuring the amount generated, and I also try to track my propane usage using just the % guage on the tank.  I have been doing this for quite a while.  I'm just trying to see if any of this means anything.  Also what about natural gas?   Thanks   Mike

Tim McElwain

Mike a theoretical

number is about a gallon per 100,000 BTU's, which is actually the amount relative to the vapors created by complete combustion which if those vapors cooled completely would come to that number. That being said the mod/con condensate must also calculate as to return water temperature and the stack temperature in the vent. So I would say it would be difficult to calculate without more sophisticated measuring devices. The stack temp and return water temp vary based on the demand placed on the system. The theoretical dew point of the flue gases is 140 ° F for gas, oil and propane. That added to the point of condensing on most boilers being around 135° F return water temperature adding the two gives 275° F which by most is considered the lowest stack we would allow on a conventional boiler systems. Mod/Cons operate at much lower temperatures which is why they condense, the lower the return water temperature and the lower the stack temperature the higher the amount of condensate based on firing rate. That presents another problem which is the modulating side of the picture which is controlled by some kind of outdoor reset or a combination of sensing both indoor and outdoor to control the firing rate of the system. Most residential systems have a 5 to 1 turn-down ratio. Taking the number 5 and dividing it into the input will give you the lowest firing rate an example 100,000 BTU's divided by 5 would give you approximately 20,000 the lowest firing rate.



As to propane being different than natural gas as to condensate the answer is in a given situation all things being equal they would produce the same amount of condensate

icesailor

Because I saw:

Because I was up late last night/morning, I happened to see a show on some channel about air filters on trucks. BIG ars trucks that move fill in western strip mining. These trucks take a few scoops of these huge shovels and move it away. The issue was that about the dust in the roads and how it would kill these engines. They are V20 Allison turbocharged diesels. The filters work on the principle of a tornado. That the inside is clean of dust because centrifugal force throws the dust to the outside of the tubes and falls to the bottom of the filter. But that the engines sucked in 50,000 gallons of air. I don't remember in how long. I was too interested in the volume of air in gallons. So the question.

How does humidity and the amount of water vapor in the intake air contribute to the condensate of a Mod/Con boiler? Colder air holds less water vapor than warmer air.

I've never seen it discussed. But I may not have been paying attention and was fixated on something else.

Tim McElwain

The cold air being more dense it

could be argued will give a better mix for combustion. MY testing and experience however show that as long as the intake air is above 10° F it does not seem to have much of an effect. I will however concede to lower temps (less than 10° F) causing some difficulty with the air gas mix and getting a good combustion condition in the fire box. This seems to be different with Mod/Con air gas mix and is less of a problem. This is a good argument for Concentric vents as the intake air would pick up some temp as it passed over the exiting flue gas products in the other pipe.



Now as to whether it would change the amount of condensate that I would believe would be a very small difference. The air (O2 and N) mixing with the gas (carbon and hydrogen (CH4 for natural) (C3 H8 for propane) producing a certain volume of water vapor, CO2, CO (trace) and Nox. The fact that you need less air for natural gas than you do for LP could make a difference but very little. The Mod/Con boilers only allow the exact amount of air to be drawn in to match the ODR demand so at lower outdoor temperatures we would be running at higher input, more air, more condensate. The reverse as we lower combustion air blower speed and BTU demand with higher outdoor temps.



Jim Davis and I had a discussion here on the Wall about this several years back if memory serves me.

Plumdog_2

Correct me if I'm wrong

but I always thought cold air was MORE dense than warm air.

Tim McElwain

You are correct Plumdog

it had been a long day I will correct the posting.

icesailor

Moisture content:

Cold air doesn't hold as much moisture as warmer air.

I guess it doesn't matter because as the air temperature drops, the amount of moisture in the air drops. It falls out as the dew point and temperature coincide. The humidity can stay the same but the water volume drops as the temperature drops.

A cubic foot of 10 degree air has less moisture and volume as the same air that is 100 degrees. If the 10 degree air is heated, it expands and the humidity and moisture content goes down.

WAIT a minute. The condensate comes from the moisture in the air, not from the boiler. It happens because the warm moist water laden air is cooled beyond the dew point as it passes through the HX. Because there is hydrogen and oxygen in the exhaust, there will be water forming.  As the latent heat is transferred to the warmer exhaust, the exhaust cools and it goes beyond the dew point. That's how Mod Cons become efficient. They remove heat from the exhaust and add it to the system fluid. Making the water condense is part of the heat/cool cycle.

SWEI

Condensate

CH4 + O2 --> CO2 + H2O

Jean-David Beyer

CH4 + O2 --> CO2 + H2O

CH4 + 2O2 --> CO2 + 2H2O

SWEI

Right

mine didn't balance.