- #1

Satonam

- 38

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- TL;DR Summary
- I'd like to understand the real-life designing process for these systems. When you search for actual components in the market, how do you tie-in the data in the spec sheet (which may have different terminology) with the values you calculated?

Greetings!

I'm a recent ME grad and I'm trying to design a refrigeration system for the first time. I understand most of the theory and calculations (I guess not, cause I wouldn't be here otherwise), but I'm having difficulty applying it to the real world. Homework problems in college often prescribe initial conditions, however, in the real world we have to determine what those conditions are before starting the problem! College never explicitly shows us how to do these things...

Anyway, after solving a problem I created with conditions I thought made sense based on research, I've decided that it might be easier if I just selected an arbitrary compressor for my calculations and see what happens. Here is the compressor I'm looking at: https://www.grainger.com/product/EMBRACO-Refrigeration-Compressor-5AHA1

At the moment, I'm mostly confused about the "Evaporating Temp" and the "Condensing Temp" listed on their webpage. Is it a limitation of the compressor? Recommendation? It doesn't even state a pressure to go with those numbers...

In my latest analysis, I have 29.75 F @ 0.28MPa going through the evaporator and into the compressor inlet. At the compressor outlet, assuming isentropic compression, I have 106.16 F @ 0.7MPa. (I converted celsius to fahrenheit in this post to be consistent with the units reported by Grainger)

I began constructing my analysis at the Compressor Inlet as State 1 of the Vapor-Compression Refrigeration Cycle. I know that, to avoid air leaking into the system, the pressure in the system must be greater than 1 atm at all times; therefore, I arbitrarily chose a starting pressure of 0.28MPa due to ignorance of possibilities. I expect to lower this pressure to a more realistic value as I learn more through the design process. Naturally, assuming an ideal system without superheating, the temperature at State 1 is taken at the saturation temperature of R-134a vapor.

The pressure at State 2 was chosen as 0.7MPa. I did light research to conclude that the hottest temperature of my currently location throughout the year is 30 C. So I chose 0.7 MPa because it was the lowest convenient pressure in the superheated charts that provided a temperature above 30 C with the same entropy of State 1, which will enable heat rejection to environment at the condenser. (Specifically, 41.2 C / 106.16 F)

Using an expansion valve, the enthalpy is assumed to be constant as the pressure drops from the condenser exit to the evaporator inlet. No subcooling.

Of course, the pressures chosen don't take into account whether a compressor exists in the market that can actually perform the task specified, this was just a dry run to get my feet wet. If I have done this correctly, the work input for this hypothetical situation is 0.9 kW / 1.2 HP with a COPr of 8.9. (Assuming an arbitrarily chosen mass flow rate of 0.05 kg/s) The ideal COPr,carnot calculated was 9, using TL = 10 C and TH = 30 C. The fact that the COPr is so close to ideal indicates I'm very skeptical of results. Haha...

With that said, I used exergy to determine the theoretical minimum power input required to remove heat at calculated rate and reject it to the environment at To = TH. The value I got was 16 kW / 21 HP, which is considerably higher than the 1.2 HP quoted earlier. Very confusing.

I'm a recent ME grad and I'm trying to design a refrigeration system for the first time. I understand most of the theory and calculations (I guess not, cause I wouldn't be here otherwise), but I'm having difficulty applying it to the real world. Homework problems in college often prescribe initial conditions, however, in the real world we have to determine what those conditions are before starting the problem! College never explicitly shows us how to do these things...

Anyway, after solving a problem I created with conditions I thought made sense based on research, I've decided that it might be easier if I just selected an arbitrary compressor for my calculations and see what happens. Here is the compressor I'm looking at: https://www.grainger.com/product/EMBRACO-Refrigeration-Compressor-5AHA1

At the moment, I'm mostly confused about the "Evaporating Temp" and the "Condensing Temp" listed on their webpage. Is it a limitation of the compressor? Recommendation? It doesn't even state a pressure to go with those numbers...

In my latest analysis, I have 29.75 F @ 0.28MPa going through the evaporator and into the compressor inlet. At the compressor outlet, assuming isentropic compression, I have 106.16 F @ 0.7MPa. (I converted celsius to fahrenheit in this post to be consistent with the units reported by Grainger)

**A Little Bit About The Values I Calculated:**I began constructing my analysis at the Compressor Inlet as State 1 of the Vapor-Compression Refrigeration Cycle. I know that, to avoid air leaking into the system, the pressure in the system must be greater than 1 atm at all times; therefore, I arbitrarily chose a starting pressure of 0.28MPa due to ignorance of possibilities. I expect to lower this pressure to a more realistic value as I learn more through the design process. Naturally, assuming an ideal system without superheating, the temperature at State 1 is taken at the saturation temperature of R-134a vapor.

The pressure at State 2 was chosen as 0.7MPa. I did light research to conclude that the hottest temperature of my currently location throughout the year is 30 C. So I chose 0.7 MPa because it was the lowest convenient pressure in the superheated charts that provided a temperature above 30 C with the same entropy of State 1, which will enable heat rejection to environment at the condenser. (Specifically, 41.2 C / 106.16 F)

Using an expansion valve, the enthalpy is assumed to be constant as the pressure drops from the condenser exit to the evaporator inlet. No subcooling.

Of course, the pressures chosen don't take into account whether a compressor exists in the market that can actually perform the task specified, this was just a dry run to get my feet wet. If I have done this correctly, the work input for this hypothetical situation is 0.9 kW / 1.2 HP with a COPr of 8.9. (Assuming an arbitrarily chosen mass flow rate of 0.05 kg/s) The ideal COPr,carnot calculated was 9, using TL = 10 C and TH = 30 C. The fact that the COPr is so close to ideal indicates I'm very skeptical of results. Haha...

With that said, I used exergy to determine the theoretical minimum power input required to remove heat at calculated rate and reject it to the environment at To = TH. The value I got was 16 kW / 21 HP, which is considerably higher than the 1.2 HP quoted earlier. Very confusing.