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Thermodynamics IILaboratory Instructions

2011 Division of Thermodynamics, Institute of Heat Engineering

HHV and LHV Measurements

Junkers Gas Calorimeter

Exercise No 4

Theory

Definitions

The Higher Heating Value (HHV) or heat of combustion is the amount of energy which is released in

the process of complete combustion of a weight/volume unit of certain fuel and cooling down all the

combustion products to the ambient temperature.

The Lower heating Value (LHV) is the HHV lowered by the heat of evaporation of all the water

contained in the flue gas.

Thus the LHV is roughly (although not exactly) equal to the amount of energy released in the process

of complete combustion of a weight/volume unit of certain fuel and cooling down the combustion

products to the flue gas dew point. It is utilised to determine energy potential of combusting fossil

fuels, as in industrial practice it is usually impossible or impracticable to cool down flue gas below the

dew point (due to corrosion risk). LHV is also popularly used to determine efficiency of combustion

systems. It should be noted however, that if LHV is used to determine energy input into a system,which does cool down the exhaust gas below the dew point (like in case of heating boilers fired with

de-sulphurised natural gas), the efficiency calculated as the energy output (water enthalpy increase

divided by fuel energy input) may go beyond 100%. Such a value, while technically correct, has no

physical meaning, yet it may be utilised to compare efficiencies of several different pieces of

equipment.

Units

Typically fuel heating values are denominated in units of energy in reference to units of weight or

normalised volume. Thus most popular heating value units (in Europe) are:

For solid fuels: kJ/kg, MJ/kg, GJ/Mg

For gaseous fuels MJ/Nm.

Please remember, that Nm normal cubic metreis a unit defining amount of gas, not its actual

volume. A normal cubic metre is directly linked to certain mass of a gas or to certain amount of its

molecules. Normal cubic metre of a gas is the amount of gas which will have a volume of exactly one

cubic metre in normal conditions. Please note that reference conditions (pressure and temperature)

used to define it may vary according to standards used in different countries. In Poland the reference

conditions (called normal conditions) are:p = 101,325 Pa, T= 273.15 K.

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The Junkers CalorimeterJunkers calorimeter is a device used for measuring LHV and HHV of gaseous or liquid fuels. Its

working principle is based on the First Law of Thermodynamics. The calorimeter consists of the

following main parts:

Fuel burner

Cylindrical combustion chamber, located directly above the burner

Water jacket surrounding the combustion chamber

Cooling water inlet with a flow-control valve

Cooling water outlet with a three way valve allowing the discharged water to be directed to

sewerage or to a measurement tank

Exhaust gas outlet

Flue gas condensate outlet.

The calorimeter is equipped with three thermometers, measuring temperatures of:

Exhaust gas

Cooling water at inlet

Cooling water at outlet.

During a measurement the burner is tuned to ensure complete combustion process and then fixed in

the bottom part of the combustion chamber. Water delivered to the calorimeter is flowing through

the external jacket and is subsequently discharged either to sewerage or to a measurement

container. Exhaust gas, cooled down to the ambient temperature by the water flow, is exhausted to

the environment. Water vapour contained in the flue gas is condensing in the process of cooling

down flue gas and is discharged through a special pipe stub installed in the bottom part of the

calorimeter.

The energy released during the combustion process is therefore at first utilised to heat up the gases

inside the combustion chamber (i.e. air and fuel). Then the gases are cooled by water and the energy

is transferred to the cooling water, increasing its temperature. If the flue gas is cooled down to

ambient temperature, then it may be assumed that 100% of released energy is transferred to the

cooling water.

The calorimeter seen as a separate thermodynamic system is an open flow-type system with

following mass inlets and outlets:

Fuel gas inlet

Air inlet

Exhaust gas outlet

Cooling water inlet

Cooling water outlet.

Additionally heat may be exchanged between the calorimeters interior and ambient air through the

calorimeter walls.

Therefore we may present the energy balance of the calorimeter as:

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0f f f a a iw iw eg eg ow ow c cm HHV m h m h m h m h m h m h Q

where:

m mass flow

h specific enthalpy

Henthalpy

HHV higher heating value of the fuel gas

0Q heat flux going OUT of calorimeter (heat exchange with environment)

and subscripts meaning is:

ffuel

a air

iw inlet cooling water

owoutlet cooling water

c condensate.

As there the water flow through the

calorimeter is continuous and

steady,iw ow w

m m m

Moreover it may be assumed that

the combined enthalpy of the fuel

gas and air is approximately equal

to the enthalpy of cooled down

exhaust gas, while the total

enthalpy of condensate is close to

zero. Also bearing in mind that

enthalpy of water may be

expressed as a product of water

specific heat capacity cand its temperature expressed in degrees Celsius t, we may therefore simplify

the equation to:

0f w iw owm HHV m C t t Q

Heat losses to the environment may be cooled by adjusting the water flow in a way ensuring that the

temperature of calorimeter external walls will be equal to the ambient air temperature (as any

potential heat transfer at this boundary is driven by a potential temperature difference). As the

CALORIMETER

BHf

HiwHow

Hc

Heg

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water does not have uniform temperature within the calorimeter, this condition is fulfilled by

adjusting water flow in such a way that the average water temperature (i.e. mean temperature

between the inlet and outlet) is equal to the ambient temperature. In such a case we may assume

that 0Q and therefore

f w ow iwm HHV m C t t

If the equation is integrated over certain time during which the calorimeter operates at steady state,

then it transforms into:

f w ow iwm HHV m C t t

Thus the higher heating value is calculated as

w ow iwf

m C t t

HHV m

where:

wm mass of cooling water which has passed through the cooling jacket during the

measurement period

Cspecific heat capacity of water

fm mass of fuel combusted in the burner during the measurement period

iwt water inlet temperature during the measurement period

ow

t water outlet temperature during the measurement period.

As in practice water temperatures vary slightly, average values from the whole measurement period

are used.

The fuel LHV is determined according to the definition. This means that it is necessary to collect the

condensate from the flue gas during the measurement period. Then the lower heating value may be

calculated as:

cLHV HHV m r

where:

cm mass of condensate collected during the measurement period

rheat of evaporation of water.

Mass of the combusted fuel is determined according to indications of the gas flow meter, which

indicates:

Volume of consumed gas

Supply gas pressure

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Supply gas temperature.

Please note that the volume of consumed gas is valid for measured parameters only! It may be

converted to normalised volume (expressed in normal cubic metres) using Clapeyrons equation.

Then a mass of consumed fuel gas may be calculated using its density.

Exercise instruction

During the exercise you will measure LHV and HHV of a 50/50 mixture of propane and butane.

Preparation

Record the ambient temperature

Open the main cooling water valve (red handle)

Check the water inlet temperature

Calculate the desirable outlet water temperature Open the gas supply valve and ignite the burner

WARNING! DO NOT OPEN THE GAS VALVE AND DO NOT IGNITE THE BURNER UNLESS

SPECIFICALLY ORDERED TO DO SO BY YOUR INSTRUCTOR! FAILURE TO COMPLY WITH THIS

REQUIREMENT IS CONSIDERED A SERIOUS VIOLATION OF HEALTH-AND-SAFETY RULES AND

MAY RESULT WITH A FAILURE TO GET A POSITIVE FINAL GRAD FROM THE COURSE!

Adjust the burner to obtain a smooth blue flame, proving that combustion is complete

Put the burner under the calorimeter. Do not attach it yet

Adjust the fine-control valve installed on the calorimeter to achieve desirable outlet water

temperature

CAUTION. The valve controls the water flow into the calorimeter. The thermometer shows

the temperature of outgoing water flow. This means that the temperature response will be

delayed. Take your time and allow the system to respond to even fine valve movements.

CAUTION. If you set insufficient water flow, then the outgoing water temperature will

rapidly increase. This might lead to destroying the thermometer. If you are unable to

contain temperature increase (i.e. if the temperature exceeds approximately 35C and

keeps growing) immediately remove the burner from the calorimeter and wait for the

temperature to drop. Only afterwards you may try again.

When the desired outgoing water temperature is achieved and stabilised, verify that the

exhaust gas temperature is equal to the ambient temperature. If it is the case, you may start

the main measurement.

Main measurement

During the main measurement you need to record the following parameters at regular intervals (e.g.

30 seconds):

Fuel gas pressure and temperature

Fuel gas consumption (measured from time zero)

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Water inlet temperature

Water outlet temperature.

At the time zero, except for recording those parameters, you need to perform two more actions:

Switch a three-way valve on the outgoing water line, so the water is discharged by the stubpipe into your measurement container

Put a wash-bottle under the drain of the exhaust gas condensate.

The measurement continues until the amount of collected condensate is sufficient to measure it with

reasonable accuracy. Please note that it may be necessary to switch cooling water containers. You

may measure amount of collected water using a scale available at the laboratory.

Upon completion of the measurement:

Switch the cooling water valve back to the sewerage discharge

Remove the wash-bottle from the calorimeter and measure (weigh) the condensate

Sum up entire cooling water consumption

Remove the burner from the calorimeter

Shut down the gas supply valve. Verify that the flame goes out

Shut down the main water supply valve

Wipe any spilled water.

In your report you should include:

General description of the test equipment

All measurement records

Calculation of HHV and LHV values.

You will receive detailed report guidelines from your Instructor.

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