Net Heating Value or High Heating Value

[controlnovice]

I'm a bit (well, ok, a lot) confused on when to use NHV vs HHV. In particular: working on a Flare project where we have to keep a minimum of 300BTU/scf of vent gas to the flare. If we are sending N2 (no BTU content), we need to supplement with Natural Gas.

The gas provider has indicated their gas (dry) is 1010 BTU/scf as tested by calorimeter. Would this be NHV or HHV?

Since we are adding the Nat Gas to be combusted, not heating something (like a boiler), do we use the NHV or the HHV of the gas in our calculation of total BTU content?

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This is normally the space where people post something insightful.

 

[zdas04]

A definition might help.

Gross Heating Value (also known as High Heating Value) is the heat given off during complete combustion of a fluid. It is measured with a caliromiter. A portion of this heat is used in the reaction to boil the water that forms as a combustion product.

Net Heating Value is a theoretical value (i.e., is not measured) that starts with gross heating value and subtracts the latent heat of vaporization of the theoretical water combustion product. I say "theoretical water combustion product" because the amount of water can not be measured and must be estimated from the reaction chemistry.

The only reason that I can think of for specifying a minimum BTU/SCF content in a flare is that the lower the HV, the higher the ignition temperature and the system might be designed in a way that less than 300 BTU/SCF gas might not reliably ignite. Just a guess. If that guess is in the right direction then you would use GHV instead of NHV.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MortenA]

David,

Im not sure that i follow your initial definition?

Isnt GHV the energy that comes from combution+condensation of water formed during the burning of the fuel?

Opposed to LHV that dosnt consider this (since many burners and e.g. a flare) does not condens the water and hence does not have a possibility of achieving GHV.

Im a bit unsure wrt to net (i may google it later )

But i do agree that its a bit strange to specify a min energy flow rate to the flare, although specifying a minimum GHV (without flow) could be usefull.

 

[zdas04]

When you burn a gas stoichiometricly it give of a measurable amount of heat. Some of that heat is used (by the fire) to vaporize liquid water that forms. The gross heating value is what you measure when you burn the gas. Combustion has a known reaction that includes creation of liquid water. Some amount of the heat will go into converting that water to water vapor (latent heat of vaporization). Net Heating Value is the measured value minus the latent heat of vaporization of the water. In other words NHV is the amount of heat available to be used, but it is a theoretical number, not a measured number.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[MortenA]

So now im confused (and its mainly langua because i do understand the differences, but im most familiar with the danish terms). There then seems to be 4 different terms (although i suspect that they mean the same in pairs:

-Upper heating value: Heat of combustion including the energy you can recover if all the water formed during the combustion is cndensed and cooled to a std. temperature. Modern natural gas burning water heater will ususally have this feater and have an efficeny close to the UHV
-GHV= Same as upper?
-NHV=Same as lower heating value? You say its a "theoretical value and i dont quite know how to understand this
-Lower heating value: The energy that can be used if you dont condense any water. Older (and newer cheap) house water heater using natural gas will usually be of this type.

I know its splitting hairs but i just want to get the english words right.

Best regards

Morten

 

[zdas04]

GHV, UHV, and HHV look to all be the same thing. I avoid the latter two terms because only the first one (Gross Heating Value) has a consistent definition across every source I have referenced.

NHV, and LHV are the same thing.

We measure GHV in a calorimeter. We use mathematical relationships to derive NHV from GHV. That is why I call it theoretical.

I'm not sure that the modern high efficiency home heaters and water heaters actually cool the exhaust stack below the dew point of the water vapor. If they did that, the exhaust stack would accumulate liquid water and freeze in cold climates and accelerate corrosion in all climates. I have a high efficiency water heater and don't see any liquid accumulation around the exhaust pipe so I don't think we're actually getting to recovery of all the heat of combustion. I'm also not sure that would be a good idea.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[25362]

I read somewhere that in very high efficiency gas fired furnaces for home heating, the combustion products are cooled to near room temperature with a significant portion of the water vapor being condensed. The exhaust temperature being too cool for a conventional chimney to carry away the combustion products, powered venting arrangements are installed.

Besides, in some countries the terms used are LCV and HCV for low and high calorific values, respectively.
The words in Spanish: poder calor[í]fico inferior [PCI] and poder calor[í]fico superior [PCS].

 

[MortenA]

Well here in Denmark (where it freezes) its very common. The last house i had the burner weas condensing - and it had a drain. Its actually clogged up once, but that just raised an alarm and then i cleaned the drain.

Weisman makes them for domestic use (think it was a Weisman)

http://www.viessmann.dk/da/products/gas-brennwertkessel/Vitodens_200-W.html

in danish sorry Cant find it in the international catalog)

Best regards

Morten

 

[MortenA]

In Danish it says that the efficiency is 98% of the high heating value.

 

[engineeringcalc]

All this issues are explaining in the following link:

http://www.herramientasingenieria.com/_english/GasNatural/GNcalc.html