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[Doggert]

I've been tasked with splitting our per $ cost on natural gas between our steam boiler and our hot oil system. Our plant is split into two sections - one primarily steam heated, the other primarily hot oil heated.

The approach I use to determine this should be simple - although none of my undergrad texts seem to give a simple "rule of thumb" answer to this. Obviously, I could spend a lot of time going back through and checking the design for every piece of equipment in the plant, determining a heat duty, checking the amount of steam/hot oil to be used, etc. - and somehow split that back across the total amount of natural gas used - but I think that may be overthinking the problem a bit much.

My problem is this - we only know the total amount of natural gas coming in. There's nothing that I can see that's monitoring how much is actually going into the hot oil heater and how much is going into the steam boiler.

Without knowing those quantities, and without going through an "undergrad" rigorous redesign of the system - are there easy "rule of thumb" equations to quickly determine the amount of natural gas needed to fire a boiler at a given capacity and needed to heat an amount of hot oil to a given temperature?

Thanks for your help.

EDIT - I've decided on a method to solve this problem. Although it's not as quick and easy as I'd intended this to be, I believe it's the most accurate.

[Adriaan]

Given that environmental factors are - roughly - equal for both systems the data you would need are;

1. stack temperature of the boiler / heater and fluegas oxygen content
2. feedwater and steam conditions for the boiler (pressures and temperature)
3. inlet and outlet temperatures for the oil heater and the oil specific heat
4. heating oil flow and feedwater (or steam) flow

From this you can calculate the energy efficiency of the boiler / heater (by comparing the energy change in the steam / oil to the energy loss in the flue gas); assume that the rate of enthalpy change of fluegas is constant (for natural gas I'd use 1.0 kJ/kg.K). Use this efficiency to give a rough value for the total energy needed for either.

By adding the used and waste energy of the boiler / heater PER UNIT of steam / oil and multiplying with the flow of each medium, then summing the results you have a total energy consumption approximation. Use the relative energy consumption (boiler energy use / total energy use or heater energy use / total energy use) as a (admittedly rough guesstimate) way to split the natural gas cost.