Condensate
Steam is comprised of two types of energy; latent and sensible energy. When steam is
supplied to a process application (heat exchanger, coil, tracer, etc.) the steam vapor releases
the latent energy to the process fluid and condenses to a liquid better known as conden
sate. The condensate contains the sensible energy from the steam vapor. The condensate
can contain as much as 16% of the total energy in the steam vapor depending on the
steam pressures.
One of highest return on investments is to return condensate to the boiler plant. As fuel
costs continue to rise, it is imperative to focus on recovering condensate in all industrial
steam operations.
Condensate contains not only water, but also boiler treatment chemicals and energy that
was transferred during the combustion process in the boiler.
Condensate therefore, needs to be returned to the boiler in order to:
o Improve energy efficiency
o Reduce chemical cost
o Reduce make-up water costs
o Reduce sewer system disposal costs
o Meet environmental regulations
Unfortunately, a large percentage of industrial plants are wasting the condensate from
the steam system and are not taking a proactive step in returning condensate to the boiler
plant. Condensate that is being returned is still losing the thermal energy due to unin
sulated tanks, uninsulated condensate pipe, valves and fittings. The “Best Practice” for
condensate is that all devices in the condensate system are insulated to prevent thermal
energy losses.
If condensate is not returned to the boiler plant, the steam system has to make-up the
loss with cold, untreated, raw water that has to be prepared for the boiler operation.
The preparation of the make-up water has a preparation cost. The make-up water is also
going to contain substantially lower BTU content that has to be raised in the deaerator or
atmospheric feed water heater. This energy addition will add even more cost to the steam
operation. The raw water has to be chemically prepared for the boiler operation, which is
an added cost to the boiler plant operation.
Condensate return tank – Designed to meet
today’s industrial standards.
STEAM SYSTEMSCondensate Return Benchmark
With today’s high energy cost we must return the highest percentage of condensate back
to the boiler plant to be reused in the boiler.
The benchmark for the optimum condensate return percentage is up to 90%. This
benchmark is possible if the plant does not have requirements of direct steam injection
for process applications.
Justification to Return Condensate (Energy Calculations)
Condensate tank totally insulated
Improve Energy Efficiency
Below is an example of a typical operating steam system:
- Average steam flow
44,000 lbs per hour
- Cost per MBTU at the plan
$15.30 (unloaded cost)
- Operation (24 hours /7 day per week) (8760 hours)
- Operating steam pressure
o Steam temperature
150 psig
o Steam total energy (hg)
1195.1
- Makeup water temperature
55ºF (13ºC)
- Makeup water BTU content (hm)
23 Btu
- Condensate return temperature
212ºF (100ºC)
- BTU content of the condensate being returned (hc)
180.33
No condensate returned to the boiler plant
- (hc – hm) = energy loss per lb
- (180.33 – 23) = 157.33 BTU per lb
- 44,000 lbs of steam = 44,000 lbs of condensate (90% return) = 39,600 lbs
- 39,600 lbs x 157.33 (BTU per lb) = 6,230,268 BTU’s
- 6.230268 x $ 15.30 = $ 95.32 (per hour cost)
- $ 95.32 x 8,760 (hours in a year) = $ 835,003.20
** $ 835,003.20 per year
The potential savings is based on the amount of energy required to elevate the make-up
water of energy content (sensible energy) to that energy level of condensate being returned
in a gravity designed condensate system. The calculation does not take into account the
savings from chemicals, water and sewer costs. It also does not consider the effect of
bringing back the condensate at higher pressures, resulting in greater savings (See Best
Practice No.8 – High Pressure Condensate Return Systems).
The above is calculated with no condensate being returned to the boiler, but most in
dustrial plants are returning at least a small percentage of condensate. Each plant should
evaluate the cost of failing to return condensate and set forth a roadmap for returning
condensate.
Condensate tank totally insulated
STEAM SYSTEMSReduce chemical cost
If condensate is returned, then the need for make-up water is reduced. Lowering the quan
tity of required make-up water will result in lower requirements for boiler chemicals.
Reduce make-up water costs
Water costs are rising everywhere and a high percentage of condensate return will reduce
the total make-up water costs.
Reduce sewer system disposal costs
Sewer system costs are directly related to the intake of water. If condensate is not being
returned, the condensate is being drained to the sewer, adding to the cost for processing
the sewer waste.
Meet environmental regulations
Environmental regulations may require drain water to be treated.Condensate returned to
the boiler process reduces the water sent to the drain and the amount of water that falls
under the regulatory control.
Steam leaks are a loss of condensate and energy
What prevents the condensate energy from being returned?
One must understand the issues that prevent the condensate from being returned to the
boiler plant so the plant can establish corrective methods.
1.Pumping condensate
Select correct condensate pumps with the proper net positive suction head (“NPSH”). A
number of condensate pumps on the market can only permit condensate temperatures of
less than 200ºF (93°C). Condensate temperatures will be close to atmospheric saturation
temperature of 212°F (100°C). Therefore, the condensate pumps must have the proper
NPSH. Failure to design the proper NPSH will result in pump cavitation and damage to the
seals and impeller in a short period of time.
2.Steam trap issues
Under-sizing and improper installation of steam traps cause the steam traps to malfunction.
Too often a short solution to the problem is to drain the condensate to sewer. Many steam
trap installations have the drain valves open to remove the condensate from the process,
therefore achieving proper temperatures.
3.Condensate line corrosion
The condensate system will produce carbonic acid as a result of excessive carbon dioxide in
the system. The highest concentration of the carbonic acid will be in the condensate return
lines because carbon dioxide dissolves in cooling condensate. The majority of condensate
lines are installed with schedule 80 steel pipe and threaded connections. The steel will
deteriorate from the condensate corrosion, but the pipe threads are typically more sus
ceptible to deterioration due to corrosion. Slowing the corrosion affects on the system,
plants should use stainless for condensate pipe and valves, and avoid the use of threaded
connections.
Steam leaks are a loss of condensate
and energy
Steam blowing to atomsphere – A loss of steam and condensate
4. Condensate system insulation
Industrial steam system components must be insulated to insure the thermal energy in the
condensate is not lost. Everything in the condensate system over 120 °F (49ºC) should be
insulated. Insulation will also help protect personnel from hot condensate system compo
nents, thus improving plant safety.
- Condensate lines
- Condensate tanks
- Valves
- Some steam trap types .
5.Leaks
Plants do have leaks from malfunctioning components in the steam and condensate sys
tem, which can contribute to loss of condensate.
6.Flash steam losses
Flash steam is lost from condensate tank vents that are venting to atmosphere. We have
addressed flash steam losses in Steam Best Practices No. 7 – flash steam.
Summary
Condensate is one of the top five items that must be targeted in a steam and condensate
system to reduce energy cost and improve reliability