Boiler Draft Measurement

Boiler Draft Measurement

What Is Draft:

Draft means a difference in pressure or differential pressure. The word draft is usually used for very low-pressure differentials up to 1000 mmwc. It is measured with the help of a transducer that converts this small differential pressure to a small electrical signal. This signal, in turn, is amplified to standard values like 4 – 20 mA, 0 – 5.0 Volts, etc., and then used for display as well as control.


Applications of Draft Measurement and control:

In two main applications draft measurement and control is essential.

1. HVAC Applications

2. Boiler Applications


Here we are talking about Boiler applications.

Measuring and controlling the furnace draft is important for two reasons.

  • To increase combustion efficiency
  • To maintain safe operating conditions.


Uncontrolled draft causes poor, non-efficient operation of a furnace (poor combustion efficiency), unstable pilot flame, build-ups of highly toxic gases like carbon monoxide, soot accumulation due to varying air-fuel ratio.

It is thus extremely important to measure and control the boiler drafts. There are different points of measurement of boiler drafts & it’s approx. readings. Such as

A. FD fan outlet (Cold Air)/Air heater inlet (The usual range is about 150 to 200mmwc).

B. Air Heater outlet (Hot Air, usually between +130 to +140).

C. Furnace Draft (It should be ideally zero but kept slightly negative, the measuring range required is +/- 50mmwc)

D. Boiler Outlet (Flue Gas) about -30mmwc.

E. Economizer Outlet ranging between -70 to -80 mmwc.

F. Air heater Outlet (Flue Gas) about -100 mmwc.

G. ID Fan Inlet (Range between -140 to -150 mmwc).

H. Secondary Air Draft for a high-pressure boiler.


Brief Function of Combustion Air:

Combustion air to steam boilers is supplied (force draft) through FD fan & Secondary air fan (pressure fan at the inlet of boiler) or using ID fan (suction fan at boiler Chimney) or by using both FD, SA, and ID fans. This combination of FD, SA, and ID fans is called balanced draft fans. In boilers using the balanced draft technic, slight negative pressure is maintained at a furnace.

In this system, one of the fans is selected to control the combustion air to suit the fuel feed and the other is assigned to take care of the proper furnace draft.

Finally, all this is necessary to maintain a continuous supply of air through the fire bed for proper combustion of fuel and to exhaust the flue gases to the atmosphere after their energy content is given at the required region.

For efficient control and monitoring purposes, fairly accurate measurement of drafts at various places as mentioned above is very important.

When most of the boiler manufacturers & users were using analog (Mechanical) draft gauges, from that time, for 20 years, “MANAS” has developed and has been supplying one of the most accurate & economical drafts measuring instruments viz. Digital Draft Gauges and blind Draft transmitters. The basic principle is based on measurement of differential pressure (very low pressure) using proper techniques and then converting the signal to standard process instrumentation signal like 4-20mA or 0-5 Vdc etc.


MANAS offers two types of draft measuring instruments.

  1. Draft Transmitter:

It is a blind two-wire field mounting type transmitter, taking inlet of draft signal in pneumatic form and delivering two-wire 4-20 mA output. They are temperature compensated to take care of any drifts due to temperature. They work on a 24 Vdc two-wire system.

The power for this unit either can be applied separately or through any digital indicator having a 24Vdc transmitter supply (Manas also can supply this product). Transmitters are usually used for low draft measurement applications like +/- 250 mmwc to 200 mmwc to measure boiler draft at various points in Flue gas path & Forced air path to identify flue gas & air path leakages, which can affect the boiler efficiency as well as combustion efficiency. It can be used for boiler control systems.


2. Draft Gauge/Indicator:

It is a digital draft indicator that takes the input as a draft signal and delivers 4-20 mA output along with the digital display. It works on the mains supply of 230Vac. It contains an inbuilt transmitter function along with an indicator.


Accurate draft control at affordable price possible: 

Both the instruments are much better compared to the mechanical gauges with analog indicators. The mechanical gauges are of no use for control purposes as they cannot deliver any signal. Manas Draft measuring instruments are very economical compared to sophisticate electronic gauge transmitters and are comparable in price with mechanical gauges. In addition, the instruments can pay you back quickly in the form of saving in energy due to perfect combustion.



   Boiler Draft Measurement

Author-  Mr.Shrikant Sahasrabudhe


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