furnace efficiency

zeke

Is it a good idea to verify furnace efficiency by using delta T and cfm in the airhandler for a known nozzle and oil pressure at the oil pump? Are the standard methods more accurate or easier to perform?
My way, for what it is worth, is to get the thermal output at the airhandler and divide it by the heating value of the fuel, or

E=cfm*density of air*(h2-h1)/heating rating of fuel*gpm
where
h2= enthalpy of air leaving heating element
h1= entering enthalpy
gpm= flow rate at nozzle( function of nozzle size and pump pressure)
Make any sense?
I realize tha tsome of these measurements may not be easy to get accurately.

Jim Bergmann_2

Heat formulas

The formula you need is as follows for a furnace.

Total Sensible Heat Formula (TSH): This is the heat added or removed that causes a change in temperature of the air without adding or removing humidity to the air.

The formula: TSH=1.08 x CFM x delta-t

Where: TSH = Total sensible heat
CFM = Airflow in cubic feet per minute
delta-t = Change in temperature across the cooling coil or heat exchanger

The constant used in the sensible heat formula 1.08 is used to convert CFM to lbs/hr and factor in .24 the specific heat of standard air (BTU/lb/°F)

Where: 1.08 = (0.24 x 60)/13.33 or 0.24 x 4.5

4.5 = 60 min/hr or (60min x 0.075 lbs/cu ft)
13.33

0.24 BTU = specific heat of standard air (BTU/lb/°F)


Heat input is BTU per gallon x GPH

You could use also

Gross Total Heat Formula (GTH): This is the total heat added (heated) or removed (cooled) from the air that is being conditioned.

The formula: GTH = 4.5 x CFM x delta-h

Where: GTH = Gross total heat
CFM = Airflow in cubic feet per minute
delta-h = change in enthalpy in Btu per pound of dry air

The constant 4.5 is used to convert CFM to lbs/hr

4.5 = 60 min/hr or (60min x 0.075 lbs/cu ft)
13.33

Where 13.33 is the specific volume of standard air (cu.ft/lb) and 0.075 is the density (lbs/cu.ft)

If the air being measured is not standard air, the air density will vary with the barometric pressure and the absolute temperature. To recalculate the air density, measure the temperature and obtain the barometric pressure use the following formula:

Air Density (lb/cf) = 1.325 x Bp/Tabs

Where: 1.325 (Constant to keep consistent units)
Bp = Barometric Pressure
Tabs = Temperature (Absolute)

Example 1.325 x 29.92/(70°F + 460°F) = 0.0748 ~ 0.075 lb/cu ft
This is how standard air density is calculated

If you were measuring air coming out of a furnace, and the air was 154° F the air density would change as follows

1.325 x 29.92/(154°F + 460°F) = 0.0645

If heated air were used in this formula, the constant would be:
(60min x .0645 lbs/cu ft) = 3.87 instead of 4.5 used for standard air.

Testo makes in instrument that will perform all of these calculations automatically and correct for air density and measure the airflow all in real time the 435, it uses a annomometer and wireless probes to measure change in enthalpy. It displays BTUH, tons, or KW directly.


Thanks

Jim Bergmann
jbergmann@testo.com