# Thread: Question on calculating box temp from suction pressure

1. ## Question on calculating box temp from suction pressure

Hey guys,

A pretty basic question about box temps here, but Ive been finding that many of the explanations given to various questions on here easily trump anything Ive found book wise from a 'user-friendly' perspective, so to speak.

If you calculate your box temp based on the PT temperature equivalent of whatever your suction line pressure is, then adding say 10* TD, then 10* for your TXV............I am under the understanding that this is the formula to get box temp from suction line pressure..........My question is related to the 10* superheat from the TXV.

I THOUGHT that I had a fairly good understanding of superheat and subcooling, but Im having a hard time around understanding what is meant by superheat at the TXV and superheat at the compressor? Is this simply the superheat that is measured at the outlet of each? If so, then I fell pretty silly for asking, but if there's more to it than this then I would love to hear some ore experienced opinions.

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I'm not going to get into the TD portion of your question. I understand it, but have some difficulty explaining it well.

Superheat.

To start, I will assume that you know how I compute Superheat. If not, then you MUST learn and understand that. You must know it as well as you know your own name. Seriously it is that important.

In refrigeration, first you measure the Superheat at the evaporator. This is done the same way that you were taught in tech school. You need to have correct Superheat at the coil to be certain that you are getting proper performance.

Now, you also need to measure the Superheat at the compressor to make sure that you are protecting the compressor. Too low at the compressor and you run the risk of slugging, too high and you're going to burn it up.

3. Originally Posted by jpsmith1cm
I'm not going to get into the TD portion of your question. I understand it, but have some difficulty explaining it well.

Superheat.

To start, I will assume that you know how I compute Superheat. If not, then you MUST learn and understand that. You must know it as well as you know your own name. Seriously it is that important.

In refrigeration, first you measure the Superheat at the evaporator. This is done the same way that you were taught in tech school. You need to have correct Superheat at the coil to be certain that you are getting proper performance.

Now, you also need to measure the Superheat at the compressor to make sure that you are protecting the compressor. Too low at the compressor and you run the risk of slugging, too high and you're going to burn it up.

Totally with you JP- I mean as far as superheat at the compressor goes, that is. Like for compressor protection. But as far as performance at the evap, I don't quite follow you.
MY understanding of evaluating evap coil performance is that of calculating subcooling- because, and please correct me if Im wrong, there should be NO superheat just before the evap coil, and there should be a recommended amount of subcooling to ensure maximum heat absorption due to utilizing every BTU possible from the latent state change between liquid and vapor of the refrigerant. So many degrees below saturation temp (subcooling) ensures that you achieve this.

If you're talking about superheat AFTER the evap coil, then isn't this inconsequential? I mean isn't the real important superheat reading just before the compressor?

I apologize if my question was confusing- Im pretty sure that based on your response it was. Basically what I was trying to ask was how to calculate box temp of say a walk in cooler, based on suction line pressure. My understanding of how to do this was to take the suction line pressure and figure out your corresponding temperature with your PT chart, add whatever your temperature difference across your evaporator is, and voila! My journeyman, however, told me to ensure that I added 10* because of the superheat from the expansion valve, and this is where I got cloudy.

I know he's on target, however, because my books from second year as well as some other resources found online all mention this- the addition of 10* for the TXV.......

And thats kind of where I'm at with this sucker....

4. Originally Posted by WestcoastApprentice
...If you calculate your box temp based on the PT temperature equivalent of whatever your suction line pressure is, then adding say 10* TD, then 10* for your TXV............I am under the understanding that this is the formula to get box temp from suction line pressure..........My question is related to the 10* superheat from the TXV.
You first need to understand what the evaporator TD represents. It's the box temp minus the temp of the refrigerant in the evaporator at saturation...AKA evap temp.

Typically for a walk-in box, the TD will be around 10 Deg F. This means if your box is at 40 Deg F, you should expect a suction pressure corresponding to around 30 Deg F. The added 10 Deg F for the superheat from the TXV doesn't enter into the picture at all here, so disregard that factor in this calculation.

If you're measuring suction pressure at the compressor and not at the evaporator, you need to account for a couple of psi pressure drop in the suction line.

Evaporator TD also determines what the relative humidity in the box will be. A 10 Deg F TD will result in a good all around RH of 80-85% RH for general useage. A lower TD, like 8 Deg F will give you around 90% RH for products requiring higher humidities. For some stuff like beer and soda, a 15 Deg TD will do fine and produce 65-80% RH, so you may find this quite common in a convenience store or beverage mart. Some applications, like a meat cutting room or a loading dock where RH isn't important may be sized with a TD up around 20 Deg F or so.

Heatcraft has an excellent presentation about equipment selection in their Engineering Manual (see p.21).

http://www.heatcraftrpd.com/products...CMisc%5CEM.pdf

5. Originally Posted by icemeister
You first need to understand what the evaporator TD represents. It's the box temp minus the temp of the refrigerant in the evaporator at saturation...AKA evap temp.

Typically for a walk-in box, the TD will be around 10 Deg F. This means if your box is at 40 Deg F, you should expect a suction pressure corresponding to around 30 Deg F. The added 10 Deg F for the superheat from the TXV doesn't enter into the picture at all here, so disregard that factor in this calculation.

If you're measuring suction pressure at the compressor and not at the evaporator, you need to account for a couple of psi pressure drop in the suction line.

Evaporator TD also determines what the relative humidity in the box will be. A 10 Deg F TD will result in a good all around RH of 80-85% RH for general useage. A lower TD, like 8 Deg F will give you around 90% RH for products requiring higher humidities. For some stuff like beer and soda, a 15 Deg TD will do fine and produce 65-80% RH, so you may find this quite common in a convenience store or beverage mart. Some applications, like a meat cutting room or a loading dock where RH isn't important may be sized with a TD up around 20 Deg F or so.

Heatcraft has an excellent presentation about equipment selection in their Engineering Manual (see p.21).

http://www.heatcraftrpd.com/products...CMisc%5CEM.pdf
Your calculation you used that derived the 30*F is exactly where I was with this whole thing until my J-man told me about the additional 10* for the txv.........me thinks I need to consult him first thing after the weekend to get him to elaborate on what he was referring to.

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Originally Posted by WestcoastApprentice
Totally with you JP- I mean as far as superheat at the compressor goes, that is. Like for compressor protection. But as far as performance at the evap, I don't quite follow you.
MY understanding of evaluating evap coil performance is that of calculating subcooling- because, and please correct me if Im wrong, there should be NO superheat just before the evap coil, and there should be a recommended amount of subcooling to ensure maximum heat absorption due to utilizing every BTU possible from the latent state change between liquid and vapor of the refrigerant. So many degrees below saturation temp (subcooling) ensures that you achieve this.

If you're talking about superheat AFTER the evap coil, then isn't this inconsequential? I mean isn't the real important superheat reading just before the compressor?

I apologize if my question was confusing- Im pretty sure that based on your response it was. Basically what I was trying to ask was how to calculate box temp of say a walk in cooler, based on suction line pressure. My understanding of how to do this was to take the suction line pressure and figure out your corresponding temperature with your PT chart, add whatever your temperature difference across your evaporator is, and voila! My journeyman, however, told me to ensure that I added 10* because of the superheat from the expansion valve, and this is where I got cloudy.

I know he's on target, however, because my books from second year as well as some other resources found online all mention this- the addition of 10* for the TXV.......

And thats kind of where I'm at with this sucker....
On a conventional refrigeration system, subcooling doesn't really enter into the equation.

You would use the sightglass to ensure a solid column of liquid to the TEV, then set the superheat with the TEV once the box reaches temp.

A high superheat value at the evaporator would tell you that the coil isn't being fed properly.

Glad to see icemeister chiming in on explaining TD.

7. Originally Posted by WestcoastApprentice
...My understanding of how to do this was to take the suction line pressure and figure out your corresponding temperature with your PT chart, add whatever your temperature difference across your evaporator is, and voila! My journeyman, however, told me to ensure that I added 10* because of the superheat from the expansion valve, and this is where I got cloudy.

I know he's on target, however, because my books from second year as well as some other resources found online all mention this- the addition of 10* for the TXV.......

And thats kind of where I'm at with this sucker....
I'll let JP handle the superheat issue...because he's just better at it.

But as for adding the 10 Deg F for the TXV superheat on top of the evaporator TD, it's just wrong. If the textbook said that, then it's wrong as well.

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Originally Posted by icemeister
I'll let JP handle the superheat issue...because he's just better at it.

But as for adding the 10 Deg F for the TXV superheat on top of the evaporator TD, it's just wrong. If the textbook said that, then it's wrong as well.
Thank you for the compliment. Coming from you it is high praise, indeed.

9. Hey guys- a million cheers and likewise thanks- having the the two of you jump in on this for me makes this website like crack for a mind like mine...........not that my mind has a thing for crack.........hmmm should I edit that or allow for some light hearted humour at my own expense..........I think I'll go with the latter.

on a serious note, Im more than happy to hear JP mention that the addition of the 10* from the txv is wrong because my wife was starting to get concerned with the level of tenacity I have been displaying in the last 72 hours trying to either re-learn what I thought was correct (and now confirmed by the both of you) or to de-bunk my j-man's comments.

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Speaking to your journeyman...

I was taught a LOT of things by a journeyman who had 30+ years in when I started that weren't quite accurate and I had to re-learn the proper way to do things.

There is a difference between 10 (or 20 or whatever) years of experience and 1 year of experience repeated over and over again.

11. Ok guys- my brain won't shut off and Im gonna have no fingernails left if I have to wait until Tuesday morning to figure this stuff out. Feel free to tell me to take a hike the moment I become annoying by the way.

I'm going to try and go back a step to make sure that I understand this kind of a medium temp-charging scenario correctly:

Okay so the REASON that you can directly correlate a refrigerant temperature (the temp of the gas in your suction line) from your suction line pressure (and not have to factor in any degrees of subcooling) is because you charge to a full site glass and then stop thus allowing for full saturation, but no more.........am I correct so far?

Okay and then the calculations regarding box temp and temp diff. are no problem for me, but I think what my j-man was referring to was the fact that the txv adds 10*F superheat, so when measuring the pressure at the outlet of the evap coil (suction line) it's neccesary to factor that in. Full sight glass means saturation temperature at whatever pressure we read, PLUS the addition of 10*F from the txv.............

So if what you guys are saying is true, and I'm not saying it's not, then why do you not have to factor that 10*F in when making your calculations based on the temp/pressure relationship at the outlet of the evap?

12. WestcoastApprentice, it's high time you applied for Pro Membership here on HVAC-Talk. You have the qualifications and more than enough posts.

There's a lot of learning to be had in those areas you can't see as a Guest.

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Originally Posted by icemeister
WestcoastApprentice, it's high time you applied for Pro Membership here on HVAC-Talk. You have the qualifications and more than enough posts.

There's a lot of learning to be had in those areas you can't see as a Guest.

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