The Complete Energy Conservation Tips




Generic Tips

Minimizing compressed air losses

To minimize compressed air losses in environments in which line are prone to leakages, reduce the air compressor discharge pressure to the lowest acceptable pressure.

Air Compressors

By a rule of thumb, every 5 DegC drop in air temperature at the inlet of an air compressor reduces the energy consumption by 1%. Thus the air inlet for compressors should be taken from a cooler point.

Drainage points in compressed air distribution network

Adequate drainage points with Compressed air drain traps should be provided in a compressed air distribution network. Water entrained in compressed air not only damages the equipments at the user end, it also develops leaks in the compressed air network.

Right Choice of Air Filters

In a compressed air network the right choice of air filters not only safeguards the health of the equipment in the network but also prevents higher power consumption in the compressor.

Compressed Air Traps

Compressed air traps should be installed on the inter coolers to automatically drain the condensed water and ensure smooth system operation.

Metering the Output of Air Compressor

Metering the output of an air compressor provides a continuous health check. This enables preventive maintenance to ensure high efficiency and lower electricity bills.

Plug Air Leaks to Save Electricity.

Air is free, but compressed air is not free. Plug air leaks to save electricity.

At part load, the most efficient compressor should be used

Of the set of air compressors, at part load the most efficient compressor should be used to save energy.

Condensate and Flash Steam Recovery

Reliable Life Cycle- Checking Quality of Returned Condensate

The returned condensate quality should be periodically checked to safeguard the boiler health.

Recover Flash Steam From Contaminated Condensate

Though contaminated condensate cannot be returned to the boiler feed tank, it is possible to recover the flash steam by having the appropriate equipments in place.

Flash steam should be separated from condensate while returning the condensate to feedwater tank.

Flash steam should be separated from condensate while returning the condensate to feedwater tank.

Returning condensate by trap pressure often results in reduced condensate evacuation

Returning condensate by trap pressure often results in reduced condensate evacuation and thus higher steam consumption.

Mixing high pressure and low pressure condensate increases the back pressure

Mixing high pressure and low pressure condensate increases the back pressure on the lowest pressure header. This impacts condensate evacuation.

Condensate should be recovered.

Condensate should be recovered. It contains 20% of the energy of steam and is formed at the same pressure and temperature as saturated steam.

Ensure Maximum condensate is returned to the feedwater system.

Partial or no condensate return will results in higher fuel and water consumption. Ensure Maximum condensate is returned to the feedwater system.

Return condensate as soon as it is formed.

Return condensate as soon as it is formed. Holding condensate in collection tanks reduces the condensate temperature via radiation losses.

Advantages of Returning Condensate to Feedwater Tank

Return hot condensate from steam using processes. Besides saving 20% of fuel bill, it reduces water usage and cuts feed water treatment bills.

Energy Efficiency- Flash steam re-utilization through thermocompressors

In continuous running plants or in equipment running on stable loads, flash steam can be recovered through a process of thermocompression and re-utilized in the plant.

Energy Conservation- Proper Utilization of flash steam

To completely realize the savings from flash steam recovery, flash steam should be utilized in an application closer to the source of high pressure condensate.

Energy Conservation- Flash Steam Recovery

To completely realize the savings from flash steam recovery, the flash steam separated should be used in applications which have a steam demand higher than the quantity of flash steam generated.

Ease of Operation- Recovering flash steam - Avoiding back pressure

Recovering flash steam returns almost 50% of the energy in condensate but care must be taken to ensure the back pressure created does not affect the process.

Open vessels containing heated fluids should be covered

Open vessels containing heated fluids should be covered to minimize losses.

Flash Steam Holds 50% of the Energy Content of the Condensate

In a mixture of condensate and flash, although flash steam is only about 10% by mass it holds 50% of the energy content of condensate. Recover flash steam to utilize this energy and thus reduce fuel bills.

Condensate is almost 20% of the fuel energy

To save energy, recover condensate. Condensate is almost 20% of the fuel energy

Steam Distribution

Pipe sizing should be done correctly to avoid losses during steam distribution

Choosing pipe size bigger than required for a particular flow requirement leads to energy and monetary losses.

Long saturated steam distribution lines should be sized on pressure drop method.

Both under and over sizing of steam mains has adverse impact on distribution. Long saturated steam distribution lines should be sized on pressure drop method.

Accumulated condensate in steam lines can lead to noisy and damaging water hammer

Accumulated condensate can lead to noisy and damaging water hammer resulting in leaks in the steam system that wastes energy. To reduce the possibility of leaks, drain condensate at regular intervals.

Safety- Installation of Safety Valve

To ensure safe operations, a safety valve should always be installed after the pressure reducing valve.

Reliable Life Cycle- Installing Isotubs on Thermodynamic Traps

'Isotubs' should be installed on thermodynamic traps to reduce distribution loss and increase the life of the steam trap.

Energy Conservation- Discharge of Thermodynamic Traps

The clouds of steam that billow up from the open discharges of thermodynamic traps are often mistaken for live steam loss, but the fact is that TD traps do not waste steam and are actually energy efficient.

Reliable Life Cycle- Back Pressure on Equipment Steam Space

Condensate return lines between the equipment drain point and steam trap should be carefully engineered to ensure minimum back pressure is created on the equipment steam space.

Reliable Life Cycle- Minimizing Back Pressure on Steam Traps

Condensate return lines between the steam trap and the condensate recovery system should be carefully engineered to ensure minimum back pressure is created on the steam trap to ensure effective condensate evacuation.

Reliable Life Cycle- Differential Pressure across Steam Traps

While selecting steam traps, the differential pressure available across the steam trap should also be considered to ensure effective condensate evacuation.

The Temperature of Furnace Oil Day Tank Should be Regulated.

Over heating the Furnace Oil Day tank can create a hazardous condition in the boiler house. Thus the temperature of this tank should be regulated.

Installation of a Vacuum breaker protects the process equipment

Opening Steam valves too quickly when the pipes are cold can lead to a phenomenon called 'Water Hammer'. Water hammer can damage the steam system by erosion and corrosion leading to leakages.

Moisture Separators Should be Insulated

In moisture separator is left un-insulated, it could induce water droplets to be formed rather than remove it. For this reason, moisture separators should be insulated.

Use Steam Flow Meters With Ability to Compensate for Change in the Density

What we do not measure we cannot control. Steam flow meters with ability to compensate for change in density due to changes in dryness fraction of steam, provide for acute measurement of utility flows.

Steam Mains should be engineered to facilitate flow of condensate by gravity to the steam trap

Steam Mains should be engineered to facilitate flow of condensate by gravity to the steam trap. This ensures adequate removal of condensate.

Effective control while de superheating steam is critical to avoid steam losses.

Effective control while de superheating steam is critical to avoid steam losses.

Redundant steam lines should either be removed or blinded

Redundant steam lines should either be removed or blinded. This saves the unnecessary fixed losses in the line.

Importance of correct application based steam trap selection

Correct application based steam trap selection on process equipments can save 7%-8% of steam consumption.

Correctly sized steam mains are critical.

Correctly sized steam mains are critical. Over sizing increases radiation loss and under sizing increases pressure drops.

Install moisture separators to maintain dryness of steam.

Water droplets (moisture) traveling at steam velocities erode valve seats and fittings leading to leaks. Install moisture separators to maintain dryness of steam.

Correct Installation of Steam Traps

Installing the trap right ensures it operates efficiently and reduces steam loss. 40% of the steam traps fail because of wrong selection and installation.

Correct Installation of Air Vents

To ensure air vents aid achieving higher process efficiencies and work satisfactorily, they should be installed above the condensate level.

Balanced Pressure Thermostatic Steam Traps Should be Used for Tracing Applications

Balanced pressure traps discharge condensate in sub cooled state which is advantageous in line tracing applications. Here along with latent heat, the sensible heat in condensate is also used to maintain fluid temperatures, leading to fuel savings.

Installing View Glass After a Steam Trap Eases the Maintenance

Installing view glasses after a steam traps allows the plant team to confirm effective working of steam traps in a closed loop system. This eases maintaining the steam traps especially on critical processes.

Steam Generation

Increase in TDS levels in the feedwater tank leads to water carry over thereby increasing fuel consumption

Moisture content in steam increases with high TDS levels in the feedwater tank due to water carry over. Wet steam reduces the process efficiency thereby increasing fuel consumption.

Reliable Life Cycle- Operating boiler close to design pressure

As the pressure of steam reduces, turbulence at the water surface increases, thereby increasing the moisture carryover in the steam. To ensure high dryness fraction of steam, the boiler should be operated close to design pressure.

Safety- Maintaining the quality of feedwater

The quality of boiler feedwater is an important determinant of safe boiler operations and it should be monitored to maintain the quality within safe working limits.

Safety- Avoiding low water level condition in the boiler

A majority of the boiler related accidents occur because of water level falling below the low level. For safe boiler operations it should be ensured that the boiler low water level alarm is functional at all times.

Safety- Avoiding low water level condition in the boiler

About 80% of boiler failures can be attributed to the either low water condition or operator error. Low water level being more controllable of the two, for safe boiler operations it must be ensured that boiler is not operated at low water level.

Safety- Avoiding low water level condition in the boiler

Most of the boiler failures can be attributed to the either low water condition. For safe boiler operations both of these areas must be kept under check.

Energy Conservation- Avoiding clinker formation inside the boiler

Large drop in the furnace draught pressure is a good indicator of clinker formation. Clinker formed inside the boiler reduces the heat transfer area for the flue gases and increases the electrical consumption of the ID & FD fans.

Safety- Air purge should be allowed to ensure fuel-rich atmosphere is not formed

To ensure safe boiler operations, in between burner trips, complete air purge should be allowed to ensure fuel-rich atmosphere is not formed.

Safeguarding Boilers against Fuel Explosion

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Safety- Ensure that fuel rich environment is not created in the furnace

To safeguard boilers against fuel explosion in the furnace, operators must ensure that especially during startup and shutdown a fuel rich environment is not created in the furnace.

Energy Efficiency- Clinker Formation in Solid Fuel Fired Boilers

Solid fired boilers should regularly be checked for clinker and the same should be removed. Clinker may reduce the boiler operating efficiencies.

Energy Conservation- Multiple Boilers for Larger Steam Loads

For larger steam loads it is advisable to opt for a battery of boilers instead of one large boiler to ensure optimum fuel consumption.

Increase the inlet air temperature to increase boiler efficiency

Increasing the inlet air temperature by 30-40degC increased the boiler efficiency by about 0.6%

Avoid High Negative Draft in Solid Fuel Fired Boilers

High negative draft in a solid fuel fired boiler carries away the unburnt fuel along with the flue gas. This leads to wastage of fuel.

Generate Steam at Higher Pressure

Generating steam at higher pressure facilitates meeting peak steam demands on fluctuating loads.

The TDS inside a boiler should be maintained at recommended levels

The TDS inside a boiler should be maintained at recommended levels else it can lead to scaling of the boiler tubes and eventually failure of the tubes which is a safety hazard.

Dissolved oxygen should be removed from the feedwater tank

Dissolved oxygen eats into iron and corrodes it. Thus dissolved oxygen should be removed from the feedwater tank else it can lead to corrosion of the tank and of the boiler tubes leading to a safety hazard.

Shell type steam boilers offer better fuel efficiencies as compared to coil type boilers.

Shell type steam boilers offer better fuel efficiencies as compared to coil type boilers.

Co-generation plant operations should be decided only after comparing the running cost to cost of electricity

Co-generation plant operations should be decided only after comparing the running cost to cost of electricity.

Injection of flash steam and condensate into the feedwater tank should be via a deaerator head

Injection of flash steam and condensate into the feedwater tank should be via a deaerator head. A deaerator ensures mixing of make-up water, condensate and flash steam to deliver uniform temperatures.

Opting for shell type boilers over coil type boilers leads to availability of good quality steam

Opting for shell type boilers over coil type boilers leads to availability of good quality steam and reduces the process steam requirements.

Air to fuel ratio should be monitored and controlled to minimize unburnts

Air to fuel ratio should be monitored and controlled to minimize unburnts. Presence of 1% unburnts represents 2.5% excess fuel consumption.

Safeguarding Boilers against Fuel Explosion

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Use shell type boiler to recover condensate.

With coil type steam boiler, condensate cannot be recovered. Condensate contains 20% of the energy in steam.

Referencing the boiler efficiency to the GCV of the fuel rather than the NCV gives a more accurate picture

Referencing the boiler efficiency to the GCV of the fuel rather than the NCV gives a more accurate picture and draws our attention to the enthalpy losses.

Efforts should be made to minimize addition of surface moisture content in fuel.

Higher moisture content in fuel leads to higher enthalpy loss. Efforts should be made to minimize addition of surface moisture content in fuel.

Maintaining the right level of water in the feedwater tank reduces the overall boiler feedwater TD

Maintaining the right level of water in the feedwater tank reduced the overall boiler feedwater TDS. This leads to reduced blowdown and the associated blowdown loss.

To deliver the same amount of energy, the flowrate of hot water will have to be 45 times than that of steam

To deliver the same amount of energy, the flowrate of hot water will have to be 45 times than that of steam necessitating higher pumping costs. Use steam in place of hot water where ever possible.

Monitor flue gas temperatures to control stack loss.

A mere 20deg c deviation of flue gas temperatures from optimum value result in 1% excess fuel consumption – monitor flue gas temperatures to control stack loss.

Co-generation plant operations should be decided only after comparing the running cost to cost of electricity.

Co-generation plant operations should be decided only after comparing the running cost to cost of electricity.

Using steam instead of thermic fluids for indirect heating is more efficient.

For the same rate of heat transfer, the surface area of heat exchanger required with thermic fluid greater that what is required with steam. With larger heating surfaces the radiation losses also proportionally increase.

Monitoring flue gas temperature gives a good indication of the boiler operating conditions

Flue gas temperatures depend on steam temperature, excess air and heat transfer efficiency. Monitoring this parameter gives a good indication of the boiler operating conditions.

Boilers efficiency should be monitored.

Boilers do not operate at rated efficiencies and the running efficiencies are found to be 3%-12% lesser than rated efficiencies. Boilers efficiency should be monitored.

At part loads the most efficient boiler should be used.

In plants where multiple boilers are used to meet the steam demand, at part loads the most efficient boilers should be used.

Poor quality of fuel leads to poor boiler efficiency.

Attention should be given to ingress of moisture during fuel handling and storage. Poor quality of fuel leads to poor boiler efficiency.

Operating boiler closer to full load improves efficiency

Radiation loss of a boiler enclosure is 1% at full load and 4% at quarter load. Operating boiler closer to full load improves efficiency.

The configuration of the co-generation system is important to ensure high efficiency

The configuration of the co-generation system is important to ensure high efficiency and better return on investment.

Energy conservation is the only avenue to reduce fuel bills.

The cost of generation of steam using solid fuel fired boilers today is greater than an oil fired boiler 10yrs ago. Energy conservation is the only avenue to reduce fuel bills.

Chemical treatment of make up water increases the TDS levels in the boiler

Chemical treatment of make up water increases the TDS levels in the boiler, leading to increased blowdown. Returning condensate reduces the TDS levels and thus blowdown, leading to fuel savings.

Avoid Frequent Changes to the Boiler Fuel Firing System

Frequent changes to the boiler fuel firing system destabilize combustion, resulting in higher fuel consumption. This leads to poor boiler efficiencies and higher fuel bills.

Higher feedwater temperature increases the boiler output

Higher feedwater temperature increases the boiler output with reduced fuel consumption.

Use saturated steam for indirect heat transfer applications.

Saturated steam has a higher heat transfer coefficient than super heated steam. Thus use saturated steam for indirect heat transfer applications.

Steam is preferable to thermic fluid as heating media.

One kg of steam is equivalent to 27 Kg of thermic fluid to deliver the same heat content. For heating applications less than 200degc, steam is preferable to thermic fluid as heating media.

Maintaining high TDS levels in the boiler drum results in water carryover

Maintaining high TDS levels in the boiler drum results in water carryover and foaming. This leads to wet steam and lower heat transfer efficiencies to the process.

Avoid over sizing the boiler.

Avoid over sizing the boiler. Over sizing leads to frequent on-off cycles in a boiler which lowers boiler efficiency.

Boiler operation parameters should be monitored continuously

Boiler operation parameters should be monitored continuously to correct operations. An S: F variation of even 0.25 translates into 1.85% excess fuel consumption.

Feedwater tank should be sized to be 1.5 times the peak steam demand.

As a rule of thumb, feedwater tank should be sized to be 1.5 times the peak steam demand.

Minimize Boiler Scaling

Fuel waste due to boiler scale may be 2% for water-tube boilers and upto 5% in fire-tube boilers. Pretreatment of feedwater, adequate blowdown and boiler cleaning minimize scaling.

High feedwater temperature drives out dissolved oxygen

Dissolved oxygen eats into iron and corrodes it leading to steam leaks. High feedwater temperature drives out dissolved oxygen, thus preventing leaks and saving fuel.

Measuring specific fuel consumption gives a true reflection of your plant efficiencies

Measuring specific fuel consumption gives a true reflection of your plant efficiencies. With steadily climbing fuel prices, monitoring plant efficiencies is integral to sustain energy savings.

Flue gas temperatures are a good indicator of excess air

Excess air in the boiler combustion leads to losses. Flue gas temperatures are a good indicator of excess air and should be monitored.

Importanceof Steam Trap Selection and SIzing

A correctly selected and sized steam trap reduces process time and simultaneously saves energy.

Every 6oC rise in feedwater temperature reduces the fuel bill by 1%.

Condensate forms at steam temperatures. If it is returned to the feed water tank, every 6oC rise in temperature reduces the fuel bill by 1%.

Black or White Smoke is an Indicator of Improper Combustion

Black or white smoke emitted from the boiler chimney indicates inadequate combustion leading to excess fuel consumption. Check boiler parameters and settings

Reliable Life Cycle- Cold starting the boiler

During a cold start, the temperature of the boiler should be raised gradually. Improper warm-up of the boiler severely harms the boiler health.

The Temperature of Furnace Oil Day Tank Should be Regulated.

Over heating the Furnace Oil Day tank can create a hazardous condition in the boiler house. Thus the temperature of this tank should be regulated.

Steam Utilization

Ease of Operation- Pressure reducing valve

The bypass loop to a pressure reducing valve should always be designed such that the safety valve and moisture separator is always in line of the steam delivery line.

Reliable life cycle - Pressure Reducing Valves

To ensure smooth operation of pressure reducing valves on steam lines, moisture separators should be installed before the pressure reducing valve.

Ease of Operation- SIngle Orifice Float Traps for Process Steam Trapping

For process steam trapping applications the single orifice float steam trap is the right choice as it ensures effective condensate evacuation and lower steam consumption.

Ease of Operation- Draining a Steam Trap

When draining a steam trap to a high level return line the trap should be sized by accounting for the back pressure created by the rise to avoid logging of condensate and chances of water hammer.

Energy Conservation- Avoid Soot Deposition on Heat Transfer Surfaces

A 3mm thick soot deposition on the heat transfer surface can increase fuel consumption by about 2.5%. It is recommended to clean the fireside to maintain optimum efficiencies.

Use Vacuum Breakers on Small Heat Exchangers to Avoid Stalling

On small heat exchangers draining to atmosphere vacuum breakers may be used to avoid "stalling".

Installation of a Vacuum breaker protects the process equipment

Installation of a Vacuum breaker protects the process equipments from damage and thus enhances safety.

Ease of Operation- Designing Drain Lines between Equipment and Steam Trap

It is recommended that drain lines between steam using equipments and its' steam trap be kept as short as possible, generally less than 10 meters.

Use Diffusers at the Point of DIscharge for Steam Traps DIscharging to Atmosphere

For steam traps that discharge to the atmosphere, diffusers should be used at the point of discharge. Diffusers dampen the velocity of condensate discharge thereby leading to safe discharge.

Use Saturated Heat for Utilization in the Process

In case the boiler is required to generate superheated steam, the temperature of steam should be brought down to saturation temperatures before utilization in the process.

Use Indirect Heating for Hot Water Generation

If hot water is generated by direct injection of steam, in a closed circulation system, hot water tanks overflow by a quantity equal to the quantity of steam injected.

Insulate oil tank in the tank farm

Insulate oil tank in the tank farm. As compared to an uninsulated tank, a well insulated tank reduced the radiation loss to one thirds!

For hot water systems, usie indirect heat exchangers in place of direct injection based systems

For hot water systems, using indirect heat exchangers in place of direct injection based systems is beneficial.

On Steam heating applications controlling the temperatures to set point avoids excess steam consumption.

On Steam heating applications controlling the temperatures to set point avoids excess steam consumption.

To reduce steam consumption, a pressure reducing station should be opted for

Manually throttling a valve to reduce the boiler steam pressure to the pressure required in the process, leads to excess steam consumption.

Strainers should be installed correctly so that they do not become a source of wetness

Strainers should be installed right to ensure it does removes the debris and does not become a source of wetness in steam.

During product drying, lumps should be broken into smaller pieces

During product drying, lumps should be broken into smaller pieces. This improves the drying rate while reducing the heat requirement.

Utility flows should be measured accurately

What we do not measure cannot be controlled. Utility flows should be measured accurately to have precise trends in energy consumption.

Using multi effect evaporators in place of single effect evaporators reduces the steam consumption by about 1/3rd.

Using multi effect evaporators in place of single effect evaporators reduces the steam consumption by about 1/3rd.

Stall increases steam consumption and process time of the heat exchanger.

Frequent cycling of the control valve between its open and close positions can be a symptom of stall in your heat exchanger. Stall increases steam consumption and process time of the heat exchanger.

Circulation pumps are not needed for hot water and thermic fluids

Steam flows in response to the pressure drop along the line. Hence expensive circulation pumps are not required as in case of hot water or thermic fluids, reducing utility bills.

Use saturated steam for indirect heat transfer applications

To deliver the same heat transfer rates as saturated steam, superheated steam requires larger heat transfer area. Use saturated steam for indirect heat transfer applications.

Stall increases steam consumption and process time

Steam trap being noticeably cooler than the temperature of the steam pipe inlet to the heat exchanger, can be a symptom of stalling. Stall increases steam consumption and process time of the heat exchanger.

For air heating equipment use heat exchangers with finned tubes

For air heating equipment use heat exchangers with finned tubes instead of bare tubes. Finned tubes reduce size of heat exchanger.

Recover Blowdown Heat

Blowdown water is at the same pressure and temperature as water in the boiler drum. Thus by recovering this heat we can save on the blowdown losses.

Identify stall condition and take corrective actions.

'Stalling' in a heat exchanger reduces the heat transfer rate wasting steam and increasing process time. Identify stall condition and take corrective actions.

Individual trapping lowers process time and reduces steam consumption.

Group trapping of batch equipments should be converted to individual trapping. Individual trapping lowers process time and reduces steam consumption.

Avoid Sagging of Steam Lines

Sagging of steam lines due to failure of supports leads to condensate accumulation, causing leaks.

Avoid Group Trapping

Group trapping of batch equipments operating at different pressures should be converted to individual trapping. Individual trapping lowers process time and reduces steam consumption.

Wet steam reduces the process efficiency

High TDS levels in the feedwater tank lead to water carry over increasing the moisture content of steam. Wet steam reduces the process efficiency increasing fuel consumption.

Small leaks cost big money

A 1/8th inch hole in steam main can waste 9 tons of fuel oil per year. Small leaks cost big money, don't ignore them.

Keeping the insulation dry reduces radiation loss and thus saves fuel

Keeping the insulation dry reduces radiation loss and thus saves fuel. Heat loss by radiation from steam pipe to water or wet insulation can be 30 times greater than that to air due to higher thermal conductivity.

Tappings for steam traps should be taken through correctly sized drain pockets.

Tappings for steam traps should be taken through correctly sized drain pockets. This ensures effective condensate removal to allow optimum process efficiencies and reduced fuel bills.

Advantages of Steam Operated Pumps

Pumping condensate near boiling point using traditional centrifugal pumps reduces condensate temperature and wastes useful energy. Instead, using steam operated pumps, which successfully pump condensate at high temperatures, results in savings.

Eliminate air steam system to ensure effective heating and thus fuel savings.

Eliminate air steam system to ensure effective heating and thus fuel savings. Air can be between 1500 and 3000 times more resistant to heat flow than steel, and 8000 to 16000 more resistant than copper.

Steam line tappings for equipment must always be taken from the top of steam pipes

Steam line tappings for equipment must always be taken from the top of steam pipes to avoid entry of condensate. This ensures optimum process efficiencies and reduces fuel bills.

Use Steam Traps to Drain Condensate

Condensate hampers heat transfer. Steam traps drain condensate and trap steam, ensuring higher heat transfer rates and hence fuel savings.

Use Steam at Lowest Possible Pressure for Indirect Heating

Using steam at the lowest possible pressure for indirect heat exchange reduces the steam required and thus the fuel consumption. This is because latent heat increases with a decrease in steam pressure.