Mechanical Engineering Exam > Mechanical Engineering Videos > Heat Transfer > Sizing a Heat Exchanger: Counter Flow
Sizing a Heat Exchanger: Counter Flow Video Lecture | Heat Transfer - Mechanical Engineering
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FAQs on Sizing a Heat Exchanger: Counter Flow Video Lecture - Heat Transfer - Mechanical Engineering
| 1. What is the purpose of sizing a heat exchanger? |  |
Ans. Sizing a heat exchanger is the process of determining the appropriate dimensions and specifications for the heat exchanger based on the specific requirements of the application. This ensures that the heat exchanger can effectively transfer heat between two fluids, achieving the desired temperature change while maximizing efficiency.
| 2. How is the heat transfer area calculated for a counter flow heat exchanger? | |
Ans. The heat transfer area of a counter flow heat exchanger can be calculated using the following equation:
A = (Q / U × ΔTlm)
Where:
A = Heat transfer area (m²)
Q = Heat transfer rate (kW)
U = Overall heat transfer coefficient (kW/m²·°C)
ΔTlm = Logarithmic mean temperature difference (°C)
| 3. What factors should be considered when sizing a counter flow heat exchanger? | |
Ans. When sizing a counter flow heat exchanger, several factors should be considered, including:
- Heat transfer requirements: The desired temperature change and heat transfer rate.
- Fluid properties: The thermal conductivity, specific heat, and density of the fluids.
- Flow rates: The mass flow rates and velocities of the fluids.
- Pressure drop: The allowable pressure drop across the heat exchanger.
- Material selection: The compatibility of the materials with the fluids and operating conditions.
| 4. How can I determine the overall heat transfer coefficient (U) for a counter flow heat exchanger? | |
Ans. The overall heat transfer coefficient (U) for a counter flow heat exchanger can be determined using the following equation:
1/U = (1/hi) + (Δx/k) + (1/ho)
Where:
hi = Convective heat transfer coefficient on the hot fluid side (kW/m²·°C)
ho = Convective heat transfer coefficient on the cold fluid side (kW/m²·°C)
Δx = Wall thickness of the heat exchanger (m)
k = Thermal conductivity of the heat exchanger wall material (kW/m·°C)
| 5. What are some common design considerations for a counter flow heat exchanger? | |
Ans. Some common design considerations for a counter flow heat exchanger include:
- Minimizing fouling: Choosing appropriate materials and design features to prevent the accumulation of deposits on the heat transfer surfaces.
- Controlling pressure drop: Balancing the heat transfer area and flow path to minimize pressure drop while still achieving the required heat transfer.
- Thermal expansion: Allowing for thermal expansion and contraction of the heat exchanger components to prevent damage or leakage.
- Accessibility for maintenance: Designing the heat exchanger to allow for easy access to clean or repair the heat transfer surfaces.
- Safety considerations: Incorporating safety features such as pressure relief valves and temperature sensors to protect against overpressure or overheating.
Text Transcript from Video
In this screencast, we're going to look at
a concentric, counter-flow heat exchanger,
which means that the hot fluid (in our case
is oil) enters the heat exchanger in a different
direction, or counter direction, to the cooling
fluid (which in this case is water). The water
will enter the heat exchanger at the other
end while the oil will exit it again counter-flow
to the water. So we're going to do a similar
analysis here, finding the length of a heat
exchanger, as we did with parallel-flow. The
only difference again is going to be that
this heat exchanger is counter-flow. We're
still going to use our governing equation,
q equals U times A times delta T log mean,
as we did with parallel-flow. And again we're
going to rewrite it in terms of our length.
If we look at our properties and our equation,
what we'll see is that everything is exactly
the same in a counter-flow heat exchanger
as it is in a parallel-flow heat exchanger
except for one term. And that term is this
delta T log mean. Delta T log mean is going
to be our delta T 1 (which refers to the difference
in temperature at the entrance of the heat
exchanger) minus our delta T 2 (which is the
delta T at the end of the heat exchanger)
divided by the natural log of delta T 1 over
delta T 2. What's different in this particular
heat exchanger is, because the fluids are
going in the opposite direction, our delta T 1
and our delta T 2 are going to be different
for a counter-flow versus a parallel-flow.
So let's take a look and see what our delta
T 1 is going to be. And looking at this picture,
we can see that our delta T 1 is going to
equal the temperature of the oil that's entering
the system minus the temperature of the water
that's exiting the system. So that means that
our delta T 1 is going to equal 100 degrees
C minus 50 degrees C, or 50 degrees. Delta
T 2 is going to be the temperature of the
oil exiting the system minus the temperature
of the water entering the system. So our delta
T 2 is going to equal 60 degrees C minus 25
degrees, or 35 degrees C. And now let's find,
using these, our delta T log mean. So this
is going to be delta T 1, which is 50 degrees
C, minus delta T 2, which is 35 degrees C,
all divided by the natural log of 50 degrees
C divided by 35 degrees C. And when we calculate
this, we find that our delta T log mean is
equal to 42 degrees Celsius. And now let's
put in numbers for our length. And for sizing
a heat exchanger using parallel-flow, we found
that our heat transfer rate was 12,786 watts,
our overall heat transfer coefficient was
38.1 watts per meter squared degrees C, our
diameter was equal to 0.03 meters, and again
our delta T log mean is 42 degrees C. L is
going to equal our heat transfer rate, divided
by our overall heat transfer coefficient times
pi times this diameter times our delta T log
mean. And when we calculate this, we find
that the needed length for our heat exchanger
is 84.8 meters. We compare this for the length
of a parallel-flow heat exchanger, which is
a 110.6 meters. Why is there a difference?
Well because the delta T, or the difference
in temperature, is greater along the tube,
you need less surface area for the same amount
of heat transfer. Therefore, we would need
a shorter length for the counter-flow heat
exchanger.
a concentric, counter-flow heat exchanger,
which means that the hot fluid (in our case
is oil) enters the heat exchanger in a different
direction, or counter direction, to the cooling
fluid (which in this case is water). The water
will enter the heat exchanger at the other
end while the oil will exit it again counter-flow
to the water. So we're going to do a similar
analysis here, finding the length of a heat
exchanger, as we did with parallel-flow. The
only difference again is going to be that
this heat exchanger is counter-flow. We're
still going to use our governing equation,
q equals U times A times delta T log mean,
as we did with parallel-flow. And again we're
going to rewrite it in terms of our length.
If we look at our properties and our equation,
what we'll see is that everything is exactly
the same in a counter-flow heat exchanger
as it is in a parallel-flow heat exchanger
except for one term. And that term is this
delta T log mean. Delta T log mean is going
to be our delta T 1 (which refers to the difference
in temperature at the entrance of the heat
exchanger) minus our delta T 2 (which is the
delta T at the end of the heat exchanger)
divided by the natural log of delta T 1 over
delta T 2. What's different in this particular
heat exchanger is, because the fluids are
going in the opposite direction, our delta T 1
and our delta T 2 are going to be different
for a counter-flow versus a parallel-flow.
So let's take a look and see what our delta
T 1 is going to be. And looking at this picture,
we can see that our delta T 1 is going to
equal the temperature of the oil that's entering
the system minus the temperature of the water
that's exiting the system. So that means that
our delta T 1 is going to equal 100 degrees
C minus 50 degrees C, or 50 degrees. Delta
T 2 is going to be the temperature of the
oil exiting the system minus the temperature
of the water entering the system. So our delta
T 2 is going to equal 60 degrees C minus 25
degrees, or 35 degrees C. And now let's find,
using these, our delta T log mean. So this
is going to be delta T 1, which is 50 degrees
C, minus delta T 2, which is 35 degrees C,
all divided by the natural log of 50 degrees
C divided by 35 degrees C. And when we calculate
this, we find that our delta T log mean is
equal to 42 degrees Celsius. And now let's
put in numbers for our length. And for sizing
a heat exchanger using parallel-flow, we found
that our heat transfer rate was 12,786 watts,
our overall heat transfer coefficient was
38.1 watts per meter squared degrees C, our
diameter was equal to 0.03 meters, and again
our delta T log mean is 42 degrees C. L is
going to equal our heat transfer rate, divided
by our overall heat transfer coefficient times
pi times this diameter times our delta T log
mean. And when we calculate this, we find
that the needed length for our heat exchanger
is 84.8 meters. We compare this for the length
of a parallel-flow heat exchanger, which is
a 110.6 meters. Why is there a difference?
Well because the delta T, or the difference
in temperature, is greater along the tube,
you need less surface area for the same amount
of heat transfer. Therefore, we would need
a shorter length for the counter-flow heat
exchanger.
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Jan 05, 2025
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