Steam as Source of Energy

Steam is used as a heat energy source in most Pharmaceutical facility because of the efficient energy transmission, transportation and relatively safe to handle considering hazardous area. The common uses of steam are as below.

• Jacketed Vessel: Used for indirect heating of process fluids where the steam is fed to the surround jacket of the vessel and condense down after heat transfer to the product in the shell. Ex WFI vessel
• Heat Exchanger: Steam is used for heating ambient air to required temperature for achieving suitable heat transfer Examples are Fluid bed dryer or tray dryer and coater, Multi column distillation unit.
• Autoclave: steam-filled chamber used for sterilization purposes, for pharmaceutical equipment & accessories
• Calorifier: for heating of water used in process application or in HVAC.

Typical smoke tube packaged boiler is majorly consisting of fuel system & burner system, feed water system and blowdown & exhaust smoke handling system.

The hot gases from the burner pass up to three times through a series of tubes to gain the maximum transfer of heat through the tube surfaces to the surrounding water. Once the water reaches saturation temperature (the temperature at which it will boil at that pressure) bubbles of steam are produced, which rise to the water surface and burst. The steam is released into the steam space above, from there it enters the steam piping system. The stop valve isolates the boiler and its steam pressure from the process or plant.

At higher pressure steam will occupy less space. Steam thus produced are transported to user point through steam piping network by pressure differentials between the generation and point of use. It is advised to reduce the pressure close to the user point. Normally pharmaceutical process uses saturated steam at 3 bar and therefore at the entry point of production facility a pressure reducing station can be planned. If some process requires higher pressure a second PRS can be planned.

As long as the amount of steam being produced in the boiler is as much as that leaving the boiler, the boiler will remain pressurized. The burner will operate to maintain the correct pressure. This also maintains the correct steam temperature, because the pressure and temperature of saturated steam are directly related. Therefore, selecting a boiler capacity for the plant matters a lot. Higher to required capacity and lower to required capacity both would impact the steam cost dearly. Lower to required capacity will impact the production which is a greater loss, hence it should be carefully selected.

FEEDWATER

The quality of water which is supplied into the boiler is important. It must be at the correct temperature, usually around 80-90°C, to achieve higher efficiency and avoid thermal shock to the boiler. Typical feed water system should have a deaerator head for efficient heating and mixing of condensate return from plant, flash steam and freshwater. It also releases the non-condensable gases like dissolved oxygen from the feedwater tank.

Boiler requires treated water as feed as untreated potable water can cause foam and scaling. The boiler then would become less efficient and the steam would become dirty and wet. The life of the boiler would also be reduced.

Typical boiler feed water quality for medium pressure boiler should be low in scaling component and preferably as low as possible in terms of TDS, silica and iron. Usually basic potable water is pass through a cation bed to produce soft water. RO water is a preferred option as it can add to life and efficiency of boiler. The feed pump will add water to the boiler when required.

For pharmaceutical application it is a good idea to collect the drain of purified water, WFI, multi column and collect as feed water to boiler. Even AHU drain can be collected but ensure that it is without contamination which can affect the boiler operation or life.

BLOWDOWN

The feedwater containing dissolved solids and suspended solids gets concentrated in the bottom of the boiler and form sludge are removed in some interval by a process known as blowdown.

?This can be done manually - the boiler attendant opens the valve for a set period of time, usually twice a shift (8 hrs).

Advanced auto blowdown system is incorporated to boiler water, that monitor the water TDS and when the set value of TDS reached the blowdown starts until a pre-determined value is achieved.

Blow down heat recovery can be adopted to recover heat from the blow down water. Normally Plate exchanger is a good option to recover heat with feedwater.

LEVEL CONTROL

The water level inside the boiler is one of the most sensitive and critical parameters and should be well controlled to avoid catastrophic consequences for very low water or to avoid wet steam to process. When water level drops too low and the boiler tubes are exposed to overheat and fail, causing an explosion. If the water level becomes too high, water could enter the steam system and upset the process.

For this reason, automatic level controls are used. To comply with legislation, level control systems also incorporate to shut down the boiler and alert attention if there is a problem with the water level.

It is a legal requirement in most countries to have two independent low-level alarm systems.

Boiler Efficiency:

?The overall boiler efficiency depends on many parameters apart from combustion and thermal conductivity. The important parameters include radiation losses, convection losses, blowdown losses, flue gas losses, water temperature number of start & stop due to load variation etc. In real practice, two techniques are usually used to find out boiler performance, namely direct method and indirect method of efficiency calculation.?

Direct Method: This is also recognized as the input-output method because it requires only the valuable output like steam and the heat input for estimating the boiler efficiency. It is the ratio of energy output to input

?η= (Energy output)/(Energy input) X 100

?Formula of direct efficiency calculation is as below.

E= [Q (H-h)/q*GCV]*100

Where,

Q= Quantity of steam generated in kgs/hr

H= Enthalpy of steam in Kcal/kg

h= Enthalpy of water in kcal/kg

q= Quantity of fuel in Kgs/hr

GCV= Gross calorific value of Fuel in Kcal/kg

Indirect Method:

The indirect efficiency of a boiler is calculated by finding out the specific losses taking place in a boiler and then deducting the sum from 100%. This process involves calculating the total heat input based on the GCV of fuel and subtracting the losses as prescribed by BS 845 or equivalent IS. The losses estimated include stack losses, radiation losses, blowdown losses, etc.

Practically the overall efficiency of steam boiler varies between 71 to 80%. The losses include are as following.

Loss associated with Flue gas is about 18 to 20 %.

Loss associated with blow down is about 1 to 3%

Radiation losses about 4%.

However, considering the distribution loss which is about 5 to 15 % the overall efficiency of the steam system can be put at 55- 80%. This is one of the area where losses could be minimized to a great extent and it depends on the operation crew.

Measures to improve steam efficiency:

·???????Maximize condensate utilization as boiler feed water. Boiler feed water tank to have deaerator head where fresh boiler feed water, condensate and flash steam are mixed together

·???????Monitor boiler feed water temperature should be around 94 Deg C

·???????Proper and adequate insulation on piping and valves etc. Should have minimum bare area exposed to atmosphere.

·???????Select good quality burner based on utilization pattern and fuel.

·???????Auto blow down with blow down heat recovery for 5TPH and above

·???????Air preheater for up to 6.5 TPH boiler and economizer for 10 TPH and above boiler.

·???????Select boiler with higher steam space to meet the fluctuating demand.

·???????Monitor oxygen in the exhaust smoke the excess oxygen should not be more than 20%.

·???????Flash steam recovery as much as possible.

·???????Steam piping sizing based on the steam quality.?

To be Continued-------