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FLUE GAS HEAT RECOVERY and DRAFT CONTROL

                                                   

                     NEW 14-PAGE BROCHURE AVAILABLE - CLICK HERE FOR A COPY

The new brochure explains the principle of operation of our ecoDENSER. It includes flue gas losses and condensation graphs, as well as other useful data regarding Excess Air, Draft Control, System Schematics and more....

Large quantities of expensive heat are wasted as hot gases escape to the atmosphere via the flue stack.
There are two basic methods used for flue gas heat recovery: non-condensing and condensing.

The respective economic benefits of these two technologies are illustrated below.

NON-CONDENSING ECONOMISERS

These are installed into the flue stack, and they usually use finned coils to pick up a proportion of the Sensible Heat of flue gas, which raises the temperature of boiler or process water pumped through the coils. By design, they avoid reducing the flue gas temperature below the gas condensation temperature (dew point) to prevent corrosion of ductwork and of the economiser itself.

 
                                          
The diagram above illustrates a typical economiser installation. It shows a boiler generating 4 t/h of steam at 10 bar g, with 80% efficiency, using 12 GJ/h of Natural Gas.
Without any flue gas heat recovery, some 20% of the energy input, i.e. 2.4 GJ/h, are lost through the flue stack.
A non-condensing economiser can reduce this loss by 2 - 4 %, in this case 3.5%, by transfering 0.42 GJ/h of the Sensible Heat content of flue gas to cold water. Often this heat content is used to pre-heat boiler feedwater.


DIRECT CONTACT CONDENSING FLUE GAS HEAT RECOVERY UNIT

These units are designed for Natural Gas fired boilers or process heaters, specifically to make use of the latent heat inherent in flue gas. Water vapour is introduced primarily by the combustion of hydrogen. In the case of Natural Gas, 2kg of water vapour is generated for every 1 kg of Natural Gas burned. This of course means the addition of the required heat of vapourisation (Latent Heat), which is not recovered unless a Condensing Flue Gas Heat Recovery Unit is employed.
The following diagram illustrates the much greater amount of heat that can be recovered with Condensing Units.

                                         
The boiler operating parameters are the same as before, showing a 20% flue gas loss without any form of heat recovery.
If a Condensing Heat Recovery Unit is installed, such as our ecoDENSER, significant savings are possible, since a large proportion of the latent heat of water vapour (steam) is now also recovered. In the example shown, this amounts to a saving of 16% of the Natural Gas bill for this boiler.
To relate the energy savings to actual dollars, assume a cost of Natural gas of $10/GJ and 8000 hours/year operation.

In the case of the Non-Condensing Economiser, the saving would be 0.42 GJ/hour x 8000 hours/year x $10/GJ = $ 33,600 PER ANNUM
SAVED.

For the Condensing Heat Recovery Unit, the saving would be 2.0 GJ/h x 8000 hours/year x $10/GJ =
$ 160,000 PER ANNUM SAVED.

                                       

This graph illustrates the heat recovery savings as a percentage of the Higher Heat Value (also known as Gross Heat Value) of Natural Gas, due to condensation of water vapour. Note that this saving due to latent heat recovery is in addition to the sensible heat which can be extracted from flue gas.

Our DIRECT-CONTACT CONDENSING UNIT has numerous other benefits, some of which are shown below.

Contact us for an obligation-free initial evaluation of your natural gas-fired boiler or process heater.

The payback for our system is usually very short if a sufficient quantity of boiler make-up or process water is available as a heat sink.

Pre-heating of combustion or process air is also a candidate for utilising the recovered heat.


                          


ADVANTAGES AND FEATURES
No heat exchanger installed in flue stack, therefore no possibility of obstruction and fouling.

Any number of secondary heat exchangers can be installed in parallel to service different plant processes requiring hot water or product.

Simple to retrofit to existing boilers or other gas-fired heaters, without the need for major flue stack modifications.

Stainless steel construction throughout ensures a maintenance-free system impervious to the slightly acidic (carbonic acid, ph ~5, about the same as beer :-) condensate of natural gas combustion.

Usually no make-up water required - in fact the system "generates" its own make-up by condensing water vapour.

Monitoring and control system designed to suit individual requirements.


  THERMGARD ecoDENSER Basic System Components

                                

     

CONTACT US FOR FURTHER INFORMATION


                           NEW!

ecoFLUE heat recovery exchanger designed for smaller boilers, dryers, ovens etc.

The heart of the ecoFLUE is a multi-tiered spiral-wound flexible stainless steel tubing. Apart from offering greatly enhanced heat transfer properties compared to plain tubing, the corrugated tubing is inherently suitable to absorb thermal expansion and contraction.



Modular design facilitates easy adaption to any existing or new installation. Currently (April 2013) ecoFLU models are suitable for burner outputs up to 500 kW, but larger units are in the planning stage.

The compact size allows installation into the tightest plant rooms.

A built-in bypass damper does away with the requirement for external bypass arrangements.

The pressure drop through the ecoFLUE is extremely small, and most forced draft burner systems will not be adversely affected due to this small backpressure. However, we can incorporate a suitable draft inducer if necessary.






DRAFT CONTROL

Air/Fuel ratio control for all boilers is set up first and foremost to provide for operational safety. A secondary, but important criteria is the optimisation of Excess Air. This is the amount of air over and above the minimum (a.k.a. stoichiometric) amount necessary for complete combustion.

Many variables are involved in real world combustion systems. Fluctuations in supply air temperature, humidity and barometric pressure, variable pressure drops across dampers and flue stacks with changes in firing rate, are just a few of the factors which affect the flow rate settings for air and fuel.

Excess Air settings must take into account these and other factors to ensure safety and economy of operation. In practice, Excess Air settings are often well above ideal conditions.

Excess Air settings can be trimmed using e.g. oxygen sensors and control equipment, and most larger boilers and furnaces are so equipped.

However, the original equipment cost, together with ongoing maintenance requirements, mitigate against the use of 02 trim systems for small and medium sized boilers.

Our Flue Gas Heat Recovery (FGHR) system will utilise that extra "safety air" over and above the ideal Excess Air setting. A large portion of the sensible heat content of "extra" Excess Air is able to be recovered in our system.

If the choice is between installing O2 trim or FGHR, it may be easier to justify the installation of a FGHR system which, as a side benefit to saving 10 - 15% of input fuel costs, also "mops up" the heat content of some "safety air", instead of installing an 02 trim system which achieves a theoretical gain of 1 or 2% in efficiency by reducing Excess Air.

Draft Control is another benefit when our FGHR system is chosen. Since an Induced Draft (ID) fan is required to overcome the slight pressure drop across our primary heat exchanger vessel, it is only logical to use this opportunity for accurate control of the flue gas pressure at the outlet of the boiler.

The motor speed of the ID fan usually included in our FGHR package will be adjusted using a Variable Frequency controller. The input will be from a draft transmitter installed in the flue gas outlet of the boiler or furnace.

By this method we are effectively creating a balanced draft combustion system, where the flue gas flow precisely matches the combustion air supply flow.
                     

Since accurate draft control eliminates one of the major variables in the combustion control system, this is a highly recommended feature which also contributes to increased overall efficiency.

For most package boilers, the burner or boiler manufacturer recommends a slightly negative draft pressure, -2 to -5 Pa (-0.01 to -0.04"H2O) being typical. However, the setpoint may be selected for any desired value, either negative or positive.

The system we promote comes from EXODRAFT in Denmark. This company has decades of experience in this field, and enjoys a significant share of the global market. Their customers enjoy the benefits of top quality equipment which is offered as a complete pre-engineered, ready to install package.

Standard features of EXODRAFT systems for boiler draft control include pre and post-purge sequencing interlocks, draft proving switches and proportional fan speed and/or damper control.

Some EXODRAFT systems have provision for control of up to 6 boilers, with the possibility of expanding the system for even larger numbers.

CONTACT US for further details on the EXODRAFT range of draft and ventilation controls.