Heat
saving calculation.
Let
us first understand how costs / savings are calculated. We know
that,
Boiler
Heat Input is the energy from fuel, and boiler Heat Output is the
energy in steam.
Boiler
Heat Input = Qf X GCV
Where, Qf
= Quantity of fuel (in kg/hr) GCV (Gross Calorific Value)
=Energy contained in fuel in kcal/kg
Boiler
Heat Output = Qs X (Hs – Hw)
Where Qs
= Quantity of steam (in kg/hr) Hs = Heat contained in steam
(Enthalpy of Saturated steam hg) Hw = Heat already present in
the water from which steam is raised
To
find out the fuel cost for liquid/gas fuels, we need to divide by
the Specific gravity (ρ) of that fluid.
This
is the basic equation we will work with for allocating fuel cost.
Cost
of steam.
Example
5.1 Let
us take a 5TPH (tons per hour) oilfired boiler, @10.5 kg/cm2g,
using DM water at an ambient temperature of 30°C.
Direct
cost First,
we calculate the Direct costs of
• Fuel •
Water • Water treatment • Electricity
Fuel
=
7945.66 Rs/hr
Assume
the boiler operates for 24 hours a day for 335 days a year which
is about 8000 hours per year. So, Cost of FO = 7945.66 x 8000
Rs/year = Rs. 635.6 lakhs/ year (A)
Water A
5TPH boiler will use 5 tons of water per hour. Assume water cost
as Rs. 10/klitre = Rs. 10/ ton of water. So, Cost of water = 5 x
10 x 8000 = Rs. 4 lakhs/year (B)
Water
treatment Assume DM water treatment cost as
Rs. 30/Klitre. So,
Cost of treatment = 5 x 30 x 8000 = Rs.
12 Lakhs/year (C)
Electricity Electricity
is used by every boiler for running its feedpumps, blowers,
controls, etc. assume electricity consumption @ 40 Kwhr. Also,
take the cost of electricity at Rs. 4.50p per Kwhr. So,
Cost
of electricity = 40 x 4.5 x 8000 = Rs. 14.4 lakhs/year
(D)
Indirect costs These are the costs of •
Capital cost of Space, Boiler cost, Depreciation •
Manpower
Manpower Assume
3 IBR boiler Operators (salary Rs. 8000/year) and one boiler
Supervisor (salary Rs. 10,000/year). Total cost = Rs. 4.1
lakhs/year (E)
Capital cost We
will take out the cost of finance of both the space and the
boiler. Space cost. 5000 sq. ft of boilerhouse space x Rs.
400/sq.ft. = Rs. 20 lakhs. Boiler cost. Rs. 50 lakhs
Finance
cost of Rs. 70 lakhs = 0.15 x 70 = Rs. 10 lakhs/year Depreciation
of boiler = 0.15 x 50 = 7.5 lakhs/year
A total Capital cost
= Rs. 17.5 lakhs/year (F)
Adding A + B + C + D + E +
F, we get Rs. 687.6 lakhs for a 5 TPH boiler/year.
Or, Rs.
137 lakhs / ton / year, or, Rs. 1,719 / TPH !!
Cost
of leaks.
Cost
of Condensate.
In
the boiler , we heat water by suppling it energy in the form of
heat. When the temperature goes above boiling point of water it
changes state & converts to steam. Steam is the form in which
this heat energy is now carried to the process. Lets say a
jacketed vessel containing some liquid which needs to be heated.
When steam is supplied to this cooler material to be heated, the
energy is transferred from steam which new changes back to liquid
state i.e it condenses. This hot water is called
condensate.
Unfortunately, in practice not all of this heat
energy is transferred. Only 75% of the total energy carried by
steam is transferred. What happens to the rest? That”s
right. It is now in the condensate. So, if the total energy
produced by the boiler is 100, units only 75 is transferred to the
process. 25 units is trapped in the condensate.
This
condensate collects in the pipes which are at a high pressure.
When condensate is drained out from steam trap to a lower
pressure, instantaneously some of the condensate reevaporates as
Flash Steam. About half of the energy carried by the condensate
can be cost in this way. So ,out of the 25 units of heat in
condensate 12.5 units just evaporates.
What
is Condensate Management? We
have already explained condensate and flash steam. When we talk of
removing condensate, recovering it and receiving flash steam as
well, this is called Condensate Management
It is easy to
figure that collecting & recovering both condensate &
flash steam is very critical not just in money terms but also to
prevent wastage.
How
Much can Condensate recovery save the client?
• Money
Saving : Of fuel
• Money
Saved : Of water Conserved
•
Money
Saved : Of chemicals
Condensate
return cuts cost of water Condensate
is after all water, and when returned to the boiler slashes water
bills from the government, or, reduces electricity costs if we are
pumping it up from a nearby river.
Condensate
is ideal Boiler feedwater As
condensate comes from pure steam it is distilled water with no
dissolved solids. ( very low TDS). If condensate is returned, the
boiler needs lesser blowdown which is a big economic consideration
as the boiler loses energy each time it blows down.
For
blowdown and TDS talk see Chemistry
/
TDS section
Not just that, because we are recycling
condensate, we save the cost of chemical treatments that we would
have to do, were we using fresh makeup water.
Condensate
adds to boiler efficiency When
cold water is used to top up the boiler, naturally the boilers
efficiency is compromised the lower the feed water temperature,
the lesser the steam a boiler produces.
The boilers “From
and At rating” falls. Cold water put into the boiler might
even lead to thermal shock.
Condensate
saves chemical costs As
condensate is pure water, and it has already been treated, it can
be used as is in the boiler. This cuts the cost of water
treatment, too.
How do we determine how much condensate
recovery saves the client in monetary terms? Simple. Condensate
returned, means fuel saved. How much this fuel saving is, can be
determined from the formula:
Where, Qc
= Quantity of condensate (in kg/hr) Hc = Heat contained in
condensate (in kcal/kg) Hw = Heat already present in the water
at ambient tenmperature (in kcal/kg)
GCV (Gross Calorific
Value) =Energy contained in fuel in kcal/kg η = Boiler
efficiency ρ = Specific gravity of liquid/gas
fuel
Example 5.2. Find the cost savings from one ton
of condensate at 100°C, 0 kg/cm2g on an oilfired boiler.
Rs.
183.5/ hour x 800 hrs = Rs.
14.68 lakhs/ year
There
is also the added savings from water and water treatment which is
calculated similar to Example 5.1.
Cost
of Flash.
If sufficiently hot condensate from
a pressurized system is released to a lower pressure, some of that
condensate will have the heat necessary to become steam. This is
called flash steam.
‘Flash steam’ is released
from hot condensate when its pressure is reduced. Even water at
room temperature of 35°C would boil if its pressure were
lowered far enough. It may be worth noting that water at 170°C
will boil at any pressure below 7 kg/cm2g. The steam released by
the flashing process is as useful as steam released from a steam
boiler.
Example 5.3 What is the total savings
from 1 TPH condensate @ 7barg on oil? First we use a flash
separator to separate out flash at 0.5 barg. The quantity of
flash separated can be found from the formula:
Where, hf1
= Enthalpy of water at the higher pressure kcal/kg hf2 =
Enthalpy of water at the flashing pressure kcal/kg hfg2 =
Enthalpy of evaporation at the flash steam pressure
% of
flash steam generated = hf (7barg)  hf (0.5 barg) / hfg (0.5
barg) = 172  111/ 532 x 100 = 11.5 % The above formula, in
fact, is used to make the graph below.
Quantity
of flash is therefore, 11 % of 1000 kg/hr = 111 kg/hr
=
Rs 178.7/hour x 8000 hrs = Rs.
14.29 lakhs/year
After
removing flash, we will also recover condensate. The quantity of
condensate available is = (1000  111 ) kg/hr = 889 kg/hr
The
savings from this can be found out similar to Example 5.2. It
comes to Rs. 13.05 lakhs/year.
The savings from flash in most cases is as much, if not more than
condensate.
