HVAC · Heat-Transfer · Problem 2

Problem & Solution

Video Synthesis

Problem: During winter operation a cooling plant reduces energy use by using a parallel flow plate and frame heat exchanger instead of its chillers.
Approach: And then you may also want to search for parallel flow because there's an important distinction to be made between heat exchang...
Key formula: formula for calculating this and it's not all that hard it's shown a bit differently in the reference handbook than t...
Calc: If the condenser water enters the heat exchanger at 40 degrees and leaves at 46 and the chilled water enters at 60 and leaves a...
Calc: So enters at 60 and leaves at 48.
Result: I'm going to just delta t b over the natural log of delta t a over delta t b and the only difference in the reference handbook is instead of writin...

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Student questions asked in live office hours about this problem

OH 72: HVAC: Heat Transfer #2

Q: You use the parallel flow heat exchanger equation, but the problem specifies a parallel flow plate-and-frame style—do I need to memorize that this equation applies regardless of heat exchanger construction?

A: The key distinction is configuration (parallel vs. counter flow), not physical construction (shell-and-tube vs. plate-and-frame). The LMTD equation applies to either construction type as long as you know whether flow is parallel or counter flow. Focus on identifying the flow configuration, not the hardware style.

OH 76: HVAC: Heat Transfer #2

Q: When sketching this problem, the 'hot side' appears to be on the chilled water side and the 'cold side' on the condenser/cooling tower side—why is this counterintuitive assignment correct?

A: In this particular heat exchanger, the chilled water is warmer than the other fluid being cooled, which makes it the hot side by definition—it's about which stream is at higher temperature at each point, not which system (chiller vs. cooling tower) you'd normally associate with 'hot' or 'cold.' The reference handbook defines T_hot and T_cold relative to each other within the heat exchanger itself. Always sketch the temperatures and let the numbers tell you which side is hot.