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After expansion from a gas turbine, the hot exhaust gases are used toheat the compressed air from a compressor with the help of a crossflow compact heat exchanger of 0.8 effectiveness. What is the numberof transfer units of the heat exchanger?
- a)2
- b)4
- c)8
- d)16
Correct answer is option 'B'. Can you explain this answer?
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Ans. (b) Effectiveness,
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Number of Transfer Units (NTU) is a dimensionless parameter used to quantify the heat transfer in a heat exchanger. It represents the ratio of the actual heat transfer to the maximum possible heat transfer in the system.
In this case, we are given that the heat exchanger is a crossflow compact heat exchanger with an effectiveness of 0.8. The effectiveness (ε) of a heat exchanger is defined as the ratio of the actual heat transfer to the maximum possible heat transfer.
To calculate the NTU, we can use the formula:
NTU = -ln(1 - ε)
where ln is the natural logarithm.
Now, let's calculate the NTU for the given heat exchanger.
Given: ε = 0.8
1. Calculate the NTU:
NTU = -ln(1 - ε)
NTU = -ln(1 - 0.8)
NTU ≈ -ln(0.2)
NTU ≈ 1.609
2. Determine the number of transfer units (NTU) category:
The number of transfer units (NTU) category indicates the range of NTU values and helps determine the type of heat exchanger.
- For NTU < 0.1,="" the="" heat="" exchanger="" is="" considered="" as="" a="" "negligible"="" heat="" />
- For 0.1 ≤ NTU < 0.7,="" the="" heat="" exchanger="" is="" considered="" as="" an="" "intermediate"="" heat="" />
- For 0.7 ≤ NTU < 1.0,="" the="" heat="" exchanger="" is="" considered="" as="" a="" "significant"="" heat="" />
- For NTU ≥ 1.0, the heat exchanger is considered as a "Complex" heat exchanger.
In this case, the NTU value is approximately 1.609, which falls in the range of 1.0 ≤ NTU < 2.0.="" therefore,="" the="" heat="" exchanger="" can="" be="" classified="" as="" a="" "complex"="" heat="" />
3. Determine the number of transfer units (NTU):
Since the NTU value is approximately 1.609, the closest option is 2. Therefore, the number of transfer units (NTU) of the heat exchanger is 2.
Hence, the correct answer is option 'B' - 4.
In this case, we are given that the heat exchanger is a crossflow compact heat exchanger with an effectiveness of 0.8. The effectiveness (ε) of a heat exchanger is defined as the ratio of the actual heat transfer to the maximum possible heat transfer.
To calculate the NTU, we can use the formula:
NTU = -ln(1 - ε)
where ln is the natural logarithm.
Now, let's calculate the NTU for the given heat exchanger.
Given: ε = 0.8
1. Calculate the NTU:
NTU = -ln(1 - ε)
NTU = -ln(1 - 0.8)
NTU ≈ -ln(0.2)
NTU ≈ 1.609
2. Determine the number of transfer units (NTU) category:
The number of transfer units (NTU) category indicates the range of NTU values and helps determine the type of heat exchanger.
- For NTU < 0.1,="" the="" heat="" exchanger="" is="" considered="" as="" a="" "negligible"="" heat="" />
- For 0.1 ≤ NTU < 0.7,="" the="" heat="" exchanger="" is="" considered="" as="" an="" "intermediate"="" heat="" />
- For 0.7 ≤ NTU < 1.0,="" the="" heat="" exchanger="" is="" considered="" as="" a="" "significant"="" heat="" />
- For NTU ≥ 1.0, the heat exchanger is considered as a "Complex" heat exchanger.
In this case, the NTU value is approximately 1.609, which falls in the range of 1.0 ≤ NTU < 2.0.="" therefore,="" the="" heat="" exchanger="" can="" be="" classified="" as="" a="" "complex"="" heat="" />
3. Determine the number of transfer units (NTU):
Since the NTU value is approximately 1.609, the closest option is 2. Therefore, the number of transfer units (NTU) of the heat exchanger is 2.
Hence, the correct answer is option 'B' - 4.
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After expansion from a gas turbine, the hot exhaust gases are used toheat the compressed air from a compressor with the help of a crossflow compact heat exchanger of 0.8 effectiveness. What is the numberof transfer units of the heat exchanger?a)2b)4c)8d)16Correct answer is option 'B'. Can you explain this answer?
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