Psychrometrics: What Does the Sensible Heat Ratio Really Mean?

[@Rushpatil]

Thank you for the great explanation. Can you please help me understand the math on how the CFM will be selected? If we were to supply apparatus dew point, I understand that it would be much less CFM. But since we cannot do that, let’s say for the load, 10,000 CFM of 52 degree air is required (based on load = 1.08*10000*(75-52)). However, why Do we not take reheat into account? Since we are reheating to higher temperature, wouldn’t we need more CFM of air to meet the load?

[@MechanicalPEExamPrep]

This is where we drift into design. Design is science and art blended together. The science says we have to meet the sensible and latent loads, but nothing more. The designer has to decide what the supply air conditions should be, the reheat, the CFM, etc. Exam questions have to be sufficiently defined so you can solve for the missing variable. In real life, design engineers make decisions because on rules of thumb and experience, and there are multiple "correct" solutions.

[@dhrupadsheth1802]

Why we took our SHR line from point 1 and not from point 2?

[@MechanicalPEExamPrep]

State 1 is the room condition, so that's the starting point for the cooling and dehumidification that we are determining with the SHR. There are a variety of possible State 2 points which could satisfy the room load, and we are interested in the process line that goes through State 1 and has the right slope and dry bulb temp. That's how we locate State 2.

[@dhrupadsheth1802]

Thank you Dan, this was helpful👍@@MechanicalPEExamPrep