Download 191006 - Components

HEAT EXCHANGERS - FOULING/FAILURES:
HEAT EXCHANGER HEAT TRANSFER:
Heat Exchanger Fouling:
- As the thickness of the scale on the tube increases, the thermal conductivity of the
scale would decrease and cause the heat transfer rate to decrease.
- Scaling on the inside of a tube it will decrease the surface area and on the exterior
surface of a tube it will increase the surface area.
- The increase in the surface area on one side of the tube is countered by the decrease in
the surface area on the other side of the tube therefore the surface area would remain
constant therefore no effect on the heat transfer.
- The change in the surface area is minor in comparison to the change in the increase in
the thickness of the scale and the decrease in the thermal conductivity of the scale
Oil/Water Heat Exchanger Tube Fouling:
- The thermal conductivity across the tubes decreases due to the scale, the thickness
increases due to the build up of the scale, and the surface area outside the tube
remains constant and is not effected by the build up of the scale.
- The inlet temperature of the oil initially remains constant based on the heat load of the
system being cooled by the lube oil.
- Takes more energy to drive the heat across the thicker scale the oil leaving the heat
exchanger is hotter than it was before the scale buildup.
- The oil heat transfer rate initially decreases which cause the oil to heat up as the
warmer oil returns to the lube oil system
Heat Exchanger Tube Failure:
- Water sources for cooling most main condenser is not treated for the same strict purity
controls as the condensate
- The pressure of the cooling water is higher than the low pressure condenser
- A tube failure (leak) will result in the high pressure impure cooling water flowing into
the low pressure pure condensate.
- Increasing conductivity of the condensate is one of the first indications of problems in
the condenser
1 p
1
2 p
2
Rate of heat transfer between two liquids:
- Increasing or decreasing the inlet temperature of the both liquids will
result in the temperature difference across the tubes remaining
constant. Therefore the heat transfer rate would REMAIN CONSTANT.
- Decreasing the flow rate of the colder liquid will INITIALLY
DECREASE the heat transfer rate. This will eventually lead to an
increase in the temperature on the hot side (assuming a constant
heat load input) and an equilibrium situation where the hot side
temperatures are greater and the heat transfer rate is the same.
- Increasing the flow rate of the hotter liquid will INITIALLY INCREASE
the heat transfer rate. Again, the new steady state equilibrium would
be lower temperatures on the hot side and the same heat transfer
(assuming a constant heat load input).
Decreasing the temperature of a cooled system using a shell-and-tube
heat exchanger:
- Increasing the mass flow rate through the cold side will increase the
heat transfer rate, lowering the temperatures on the “cooled” side
until such time as the temperature differences between the two fluids
return to a point where the total heat being rejected is again equal to
the heat load.
- The effect will be the same both initially and in the final equilibrium
state
Changing the mass flow rate of the oil:
- If the mass flow rate of the oil increases the oil spends less time in
contact with the cooling water.
- This will reduce the amount of cooling per pound mass of oil, which
INCREASES the oil outlet temperature.
- However, heat transfer rate still increases
Mass flow rate of the cooling water verses mass flow rate of the oil:
- If the mass flow rate of the cooling water flow rate is greater than lube
oil flow rate the differential temperature of the oil would have to be
higher than the differential temperature of the cooling water.
- If the mass flow rates in the lube oil side and cooling water sides were
equal and the fluid specific heats were equal, then the differential
temperature would have to be equal
HEAT EXCHANGERS - MORE HEAT TRANSFER:
Condenser Operations - Latent Heat of Condensation:
- Maintaining circulating water temperature and flow rate while lowering the steam flow
would allow the circulating water to remove more heat from the condensed steam.
- Removing more heat from the condensed steam would lower the saturation pressure
- A lower value for the saturation temperature would result in MORE Latent Heat of
Condensation and a lower value for the condensate depression.
“Condenser vacuum improves” OR Absolute Pressure Lowers:
- This would correspond to a lower saturation pressure and temperature.
- At a lower saturation pressure it takes more heat removal to condense a lbm of steam
- With a constant heat removal (circulating water mass flow and inlet temp constant)
then there is a lesser heat removal capacity per lbm left to subcool the liquid;
subcooling decreases.
- An increase in vacuum from 2 psia to 1 psia we see that while we lose 14° subcooling
(equal to change in hfg since subcooling 1 lbm one °F takes one BTU),
Tsat decreases 25°.
- Therefore condensate temperature actually decreases 11° (14° less subcooling but a
25° lower starting point).
Turbine Exhaust Steam Quality:
- This blanketing of the tubes with the noncondensable gas with reduces the cooling
capability of the condenser.
- Saturated steam in the condenser is not cooled as much as before.
- HIGHER saturation temperature and a higher saturation pressure.
- Less steam is cooled down after the air inleakage, more steam remains as a vapor,
resulting in a HIGHER quality.
Mass of the Vapor
Quality =
Mass of the Vapor + Mass of the Liquid
m c ∆T = m c ∆T
EXAMPLE QUESTION:
Refer to the drawing of an operating water cleanup system. All valves
are identical and are initially 50% open (see figure below).
To lower the temperature at point 7, the operator should adjust valve
__________ in the open direction.
A. A
B. B
C. C
D. D
A - incorrect: increases the flow of warm
water, minimizing time Point 7 is in
contact with cooling water.
B - incorrect: increases the flow of warm
water, minimizing time Point 7 is in
contact with cooling water.
C - incorrect: increases the flow of warm
water, minimizing time Point 7 is in
contact with cooling water.
D - CORRECT: opening Valve “D”
increases the flow rate of the cooling
water. This increases the amount of heat
transfer in the H/X and the temperature at
point “7” will lower.
HEAT EXCHANGER OPERATION/CONCERNS:
Air and Non-Condensable Gases:
- The presence of air and non-condensable gases greatly reduces condenser efficiency
because the steam must diffuse through a film of non-condensable gas before reaching
the condensing surface.
- A temperature gradient is created as the gases blanket the condenser tubes.
- This results in less cooling of the steam and therefore, a higher backpressure on the
turbine (lower vacuum), reducing overall plant efficiency.
- This results in an INCREASE in temperature of the water collecting in the condenser
hotwell.
- For example from the Mollier Diagram, if 900 psi steam enters an ideal turbine and
exhaust to a 1 psi condenser the specific enthalpy is approximately 780 Btu/lbm. If the
same 900 psi steam exits an ideal turbine to a condenser at 2 psi the specific enthalpy is
about 812 Btu/lbm. An increase of 32 Btu/lbm that is not available to do work in the turbine.
Heat Exchanger Thermal Shock:
- A change in temperature that induces excessive thermal stress on heat exchanger
components.
- Thermal shock occurs when the temperature of the fluid in a system suddenly increases
or decreases, causing a differential expansion or contraction of the metal.
- Thermal stresses can be minimized by slowly valving in the cooler liquid first, then the
warmer liquid when placing a heat exchanger in service, and the opposite order when
taking one out of service.
EXAMPLE QUESTION:
A liquid-to-liquid heat exchanger containing trapped air on the shell side will be less
efficient because the air...
A. causes more turbulent fluid flow.
B. increases the differential temperature across the tubes.
C. reduces the fluid contact with the heat transfer surface.
D. causes pressure oscillations.
A - incorrect: Turbulent flow actually increases heat transfer
B - incorrect: If ∆T goes up, heat transfer increases
C - CORRECT: Air and non-condensable gases will reduce efficiency because steam must
diffuse through air to reach tubes. Also, gases blanket tubes reducing heat transfer,
raising backpressure in condenser, which also reduces efficiency.
D - incorrect: Air bubbles do not cause pressure oscillations (if anything they dampen them)
SOLVING HEAT TRANSFER EXAMBANK PROBLEMS
- One key to solving many of the exambank questions is to play close attention to the
stem assumptions. Some of them do not follow normal heat transfer parameter
changes.
- Another key is to use a combination of the Q = mcp∆T and Q = UA∆T equations, keeping
in mind that the ∆T in the Q = UA∆T equation is TAVE-HOT - TAVE-COLD. It is actually the
log mean average, but no current questions require calculating those values.
- When looking at the relationship between mass flow rates and delta-T’s of a hot and
cold system, keep in mind that the ratios of mass flow rates between the liquids is
inversely proportional to the ratios of the delta-t’s.
- For example, in the following steady-state heat transfer process:
. .
.
.
.
.
is 20oF, the m
.
mhotcp(∆T)hot = mcoldcp(∆T)cold
if the ∆Thot is 30oF and the ∆Tcold
cold must
.
be 3/2 as much as the mhot
OPS Generic Fundamentals - PWR
191006 - Components - Heat Exchangers
Rev Date: May 9, 2016
For Training Use Only