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HRR

HISTORY

The measurement of Heat Release Rate (HRR) is of vital importance in the world of fire science.  HRR is the single most important parameter describing the size of a fire. The science of measuring the HRR of a fire is called Fire Calorimetry. Today, Heat Release Rate is used to regulate various products in the commercial products industry.

 

Heat Release Rate is defined as the amount of heat produced by a burning object during a specified time interval. Heat from a fire is generally composed of  a convective plume (Hot Smoke) and a radiative component (Infrared radiation from the actual fire).

 

Prior to 1982, various schemes were used to estimate the heat release rate of fires using Mass Loss Methods and Substitution Methods.

 

Mass Loss Method

Knowing the heat of combustion of a homogeneous material allowed early researchers to estimate the heat release rate by measuring the mass loss rate of the burning item.  One simply measured the mass loss rate (kg/s) of the burning item, multiplied this by the effective heat of combustion (MJ/kg) and an efficiency factor (unit-less) to calculate the heat release rate in terms of kilowatts (or BTU’s per second using a conversion factor).  The problem with this is that this only works for homogeneous materials (wood, plastics, liquid fuels, etc. – not real world items such as furniture and other items consisting of an assembly of materials.

 

Substitution Method

Researchers sometimes used the Substitution Method for estimating the HRR of real world items involving many different materials.  The method involves burning the item of interest and passing the hot gases through a collection stack with thermocouples.  A second burn test is then performed using a gas burner to “replicate” the temperature curve.  The flow rate of fuel is then converted to HRR via a simple fuel flow/heat of combustion calculation to determine the HRR of the gas. The simple fact that two burns re required to measure the HRR of them item of interest made this scheme cumbersome – at best.

 

 

Oxygen Consumption Method

 

In 1980, an important paper was published by Huggett (prompted via liquid and gas hydrocarbon fuels research by Thornton in 1917) that showed that many solid materials produce a relatively fixed amount of heat (MJ) for every mass (kg) of oxygen consumed. This prompted the development of the Oxygen Consumption technique (1982 – Babrauskas et-al) at the National Institute of Standards and Technology (NIST – then called NBS – National Bureau of Standards).  The technique is so simple it can be described in one paragraph:

 

The procedure involves burning the item of interest under a collection hood connected to a blower. The blower is at one end of an exhaust duct producing a “suction” flow through the duct.  Only three measurements are required to calculate the Heat Release Rate.  One simply measures the air speed, temperature, and oxygen concentration in the duct to calculate the HRR with 95% accuracy.  This is because the driving factor in the equation uses the term Huggett discovered. E = 13.1 MJ/kg(O2) which is accurate to within 5% for a wide variety of materials (wood, fuels, plastics, fabrics, foams, paper, etc…). 

 

In 1998, Trevino, Janssens, and Grand developed a novel calibration procedure for Oxygen Consumption Calorimeters which is used in many fire standards in NFPA and ASTM.  The new procedure simplified estimating the Calibration Factor which requires showing the HRR formula as follows:

 

HRR = E x C x f(O2, dP, T)

 

Where E = 13.1 MJ/kg, C = Calibration factor, and f is a function involving the three main measurement variables (velocity probe pressure, Oxygen Concentration, duct flow temperature). The calibration steps are: 

  •  C = 22.1 x A (Duct area sq m)
  •   Weigh Fuel Tank
  •   Burn Propane at 40 and 160 kW
  •   Weigh Tank Again
  •   THR (theoretical) = Hc x Wt Loss (Hc = 46.54 MJ/kg)
  •   Cnew = C x THR(th)/THR(msr) or inverse depending on Hi/Lo measurement 


 

Testing Pass Fail Criteria

 

The list below shows some of the limits imposed by the codes and standards fire labs use to test and certify products:

 

  •  Room Burns – 1000 kW (1MW)
  •  Chairs – 80 kW
  •  Mattresses – 200 kW
  • Speakers – 100 kW
  •  Signage – 100 kW
  • Pipe Insulation – 200 kW;  EB4013 or 300 kW NFPA
  • Garage Doors – 250 kW
  •  Decks 25 kW/sq ft 
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Arthur Mack,
Mar 14, 2012, 8:35 AM