Effective Desuperheating
The potential to reduce the degree of outlet superheat is limited by the capability of control system. Almost all desuperheaters are used to reduce the steam (or gas) temperature as close to saturation as possible.
Most desuperheater models (types) are actually capable of achieving saturation or close to the saturation. The caveatis that these units must be controlled precisely to prevent flooding of the whole system. The limitation of the control system is its sensitivity range. Consider a system with a controlled outlet temperature set at 1 Deg.C above the saturation, where the controls are designed to maintain +/- 1 Deg.C.
The cooling water injection into the steam will continue till the temperature falls to saturation.. Then only the controls will reduce the cooling water injection. Due to the delays inherent in the control system (specifically introduction of derivative band) which is not in scope of desuperheater manufacturer, cooling water will continue to flow at the design rate for a short time. In that small amount of time, the saturated steam will be allowed to condense. Condensing vapor creates a vacuum, which sucks in more vapor, which too continues to condense. This results in loss of control, downstream equipment may be damaged & the downstream processes may be severely affected. Due to these reasons, we have opted to not guarantee desuperheating lower than 4 – 5 Deg.C above the saturation. The downstream processes do not suffer much from such small amount of superheat.
Recommendations for efficient working of DSH
• Minimum straight length at outlet should be 4 mtrs.
• Minimum distance of Temperature Sensor from the point of water injection should be 10 to 12 mtrs.
• Water pressure should be minimum 7 bar more than the steam pressure.
• Maximum recommended pressure difference between steam and water is 45 bar. If it exceeds 45 bar then install pressure control valve in waterline to reduce water pressure within limits.
• Spray water should be very clean (equivalent to boiler feedwater).
• It is recommended to install a strainer of 0.8 mm mesh before water control valve.
• Spray water should be very clean (equivalent to boiler feedwater).
• Instrument quality air is required.
• Minimum controllable temperature is Saturation temperature + 7ºC
• Minimum inlet velocity should be 15mts/sec for FND and 7mts/sec for VND.
• It is necessary to keep the steam flow rates (for max. and min conditions) as practical as possible. Else the turndown ratio increases unnecessarily which necessitates the selection and use of a combination of nozzles in the same spray cylinder
Formula for Desuperheater (water quantity calculation)
Mw = Mst x (h1-h2)
(h2 -hw )
where
Mw= Water Injection mass in kg/hr
Mst = Inlet Steam Mass in kg/hr
h1 = Enthalpy of Inlet Steam in Kcal/kg
h2 = Enthalpy of outlet steam in Kcal/kg
hw = Enthalpy of injection Water in Kcal/kg
Formula to calculate Δ P across nozzle
Δ Pn = ( Kvn x 31.6 )2 x δw
(Qw)2
Where
Δ Pn = Pressure drop across nozzle in bar
Kvn = Flow co-efficient of nozzle in m³/hr
Qw = Water quantity m³/hr
δw = Density of water in Kg/m³
Outlet pressure of water control valve = Steam pressure + Δ Pn
Save fuel with optimized process control
The following example will help you in understanding how your optimize process control can save fuel and in turn save investment cost. Imagine a situation where the controlled outlet temperature. is higher
than the saturation temperature and the said steam is to be used for heating application. It is absolutely necessary that for such applications the outlet steam temperature should be as close as possible to its
saturation temperature. The sole reason for this is the saturated steam is the best conductor of heat and as the degree of Superheat increases, the heat transfers become inefficient.
A typical example is as below
Inlet Steam Press (P1) : 10 Bar A
Inlet Steam temperature (T1) : 300 Deg. C
Outlet Steam temperature (T2) : 186 Deg. C
Steam Flow Needed to achieve above : 10000 Kgs/Hr
Inlet steam Enthalpy (H1) = 3053 KJ/Kg
Outlet steam Enthalpy (H2) = 2792 KJ/Kg
Total Ideal Enthalpy transfer : Mst x(H1-H2) = 2610000 KJ
Needed (Mst) (per Hour)
If the outlet steam temperature is not controlled properly & say the same is 15 deg.C higher than Saturation temperature of 180 deg.C, then the steam consumption shall be 10900 Kgs/Hr to achieve the same enthalpy transfer of 2610000 KJ. This means that you will end up spending 900 kgs/Hr steam extra due to the system inefficiency. Considering a typical coal fired boiler, the steam costs approx Rs. 1/- per Kg, the losses each hour shall be in the region of Rs. 900/- per hour. Hence Efficient & accurate Desuperheating is absolutely necessary, which will avoid such losses.