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Post up your box size/ location and product storage needs information.

Originally Posted by chilliwilly

Thank's pecmsg

Do you have any other info with a lot more in it?

In the fine print it gives a few more Calc s.
The link Ice posted from heatcraft gets into details!

Thank's pecmsg

Do you have any other info with a lot more in it?

The Sizing Chart is just as i said for general information only and yes there is a lot more to it.

Thank's for the link pecmsg, I don't mean to sound ungrateful but that link only appears to be for heat load calcs/box sizing but doesn't cover the stored product loads heat of respiration, and the other link looks like its for first year apprentices and trainees.

It's the application process that follows the heat load calcs that I'm after, where the component selection and sizing are done and there is an understanding being offered regarding the process. Rather than just the monkey see monkey do course that I did some years ago that still left me scratching my head afterwards.

Icemeister, I'll have a look at your website and heat craft link more in depth this evening.

BaLloyd thank's I'll have a look at that website later.

If you want more in depth engineering style talk, search Google for the refrigeration engineers forum. They tend to get pretty deep on topics over there.

Originally Posted by chilliwilly

Thank's both of you.

There isn't sufficient information when it comes to component selection, I attended a short course on design and selection some years ago but still came away scratching my head, even at college the subject wasn't really covered.

Does anyone have any pointers regarding system design and selection which focuses more on refrigeration engineering, other heat load calcs which only involves adding up the general factors such as sun path data, seasons, ambience, envelope, and other internal factors etc.

I have searched for some years now but haven't made much progress, I'm only left to think it through and develop my understanding from experience and coming on here.

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Probably the best resource for doing load estimates and equipment selection is the Heatcraft Engineering Manual:

http://www.heatcraftrpd.com/PDF/Misc/EM.pdf

The first 20 pages deal with the load calculations for typical walk-in coolers and freezers. Its taken from the method published in the ASHRAE handbook. It's the longhand way of determining the load and is important to understand and do a few of these to get a better feel for the basics. After that, there are numerous computer applications available online from the major equipment manufacturers.

Starting on p.21 the Heatcraft manual has a very good section for component selection or how to match up a condensing unit with an evaporator coil. Again, many of the online programs provided by manufacturers will also select equipment based on the calculated load, but it's still good to have a knowledge of where these selections come from so you have a better seat of the pants feel for what you're doing.

Now I've heard this guys pretty good

https://icemeister.net/backroom/

Originally Posted by chilliwilly

Thank's both of you.

There isn't sufficient information when it comes to component selection, I attended a short course on design and selection some years ago but still came away scratching my head, even at college the subject wasn't really covered.

Does anyone have any pointers regarding system design and selection which focuses more on refrigeration engineering, other heat load calcs which only involves adding up the general factors such as sun path data, seasons, ambience, envelope, and other internal factors etc.

I have searched for some years now but haven't made much progress, I'm only left to think it through and develop my understanding from experience and coming on here.

For simple box load calculations:

Thank's both of you.

There isn't sufficient information when it comes to component selection, I attended a short course on design and selection some years ago but still came away scratching my head, even at college the subject wasn't really covered.

Does anyone have any pointers regarding system design and selection which focuses more on refrigeration engineering, other heat load calcs which only involves adding up the general factors such as sun path data, seasons, ambience, envelope, and other internal factors etc.

I have searched for some years now but haven't made much progress, I'm only left to think it through and develop my understanding from experience and coming on here.

Yeah, what he said !!

Originally Posted by chilliwilly

I have visited site this afternoon and they have decided to put a new system in due it being in a poor state and badly maintained, and 404 a becoming obsolete in the UK sometime soon.

However I would still like to continue with this thread to further clarify my thoughts and understand compressor sizing and selection if you lads don't mind.

So considering what pecmsg is saying, its all about the pressure increase that will require a higher volumetric space or it will reduce the mass flow of the specific volume per stroke.

In my mind then, the sweep volume of a compressor delivering a specific mass flow at a certain temp and pressure at one speed (LBP), is now subjected to a higher temperature and a proportional higher pressure at the same sweep and speed MBP or HBP). The back pressure will reduce the useful suction sweep causing a reduction in the compression sweep, and will take longer to shift the heat energy due to the decrease in mass flow and in turn increase the risk of the compressor being damaged.

Thinking about it further, the conditions are the same as when a system first starts up from being switched off but doesn't reach its set point until its been running for some time, or never does because its mismatched due to the conditions mentioned. And may require a larger volume sweep at the same speed, or the same volume sweep at a higher speed to compensate.

But why are compressors used for low temp applications physically larger if they don't require the same sweep as a compressor used for higher temp applications. Also is an EPR fitted to a mid temp evap unit on a multi temp system on a single condenser to prevent the common suction line from going prematurely high and increasing the back pressure, rather than it being pumped down when the low temp evap unit is calling for cooling.

Compressor size and the horsepower necessary to drive it involves a lot more than just pressures.

Each refrigerant has its own set of thermodynamic properties which are different from other refrigerants. Just as their boiling points at atmospheric pressure are different, the amount of heat it must absorb to boil one pound of refrigerant liquid...ie, its specific refrigerating capacity...is also different than the others. Other factors that need to be taken into account is the refrigerant's density as a liquid and as a vapor at various pressures and temperatures and its reciprocal cousin, specific volume.

From these values we can determine what the system's mass flow in lbs per minute of the refrigerant must be to realize the capacity we need. This calculation will give us a cubic feet per minute number that we now can use to size the compressor. But the compressor isn't 100% efficient at pumping so we apply a volumetric efficiency factor, something in the range of 70-80% to give us the compressor's required volumetric displacement...or swept volume as you refer to it.

Finally we can take our numbers to determine what work (horsepower) is required to do this refrigerating. It's quite involved but still not as precise as what you would get by simply opening a compressor manufacturer's catalog. They've done all the calculating for you and have run tested the compressor so there's no guessing as to the volumetric efficiency of the design.

To answer your question in the original post, I probably would have gone with an EPR at the evap. Because I have seen "multi application" evaps on a single compressor set up that way. Not all evaps would have the EPR's, only the higher temp evaps.

To answer your question about why a larger compressor for low temps, it has to do with the reduction in capacity the lower the temp that you go.

I have visited site this afternoon and they have decided to put a new system in due it being in a poor state and badly maintained, and 404 a becoming obsolete in the UK sometime soon.

However I would still like to continue with this thread to further clarify my thoughts and understand compressor sizing and selection if you lads don't mind.

So considering what pecmsg is saying, its all about the pressure increase that will require a higher volumetric space or it will reduce the mass flow of the specific volume per stroke.

In my mind then, the sweep volume of a compressor delivering a specific mass flow at a certain temp and pressure at one speed (LBP), is now subjected to a higher temperature and a proportional higher pressure at the same sweep and speed MBP or HBP). The back pressure will reduce the useful suction sweep causing a reduction in the compression sweep, and will take longer to shift the heat energy due to the decrease in mass flow and in turn increase the risk of the compressor being damaged.

Thinking about it further, the conditions are the same as when a system first starts up from being switched off but doesn't reach its set point until its been running for some time, or never does because its mismatched due to the conditions mentioned. And may require a larger volume sweep at the same speed, or the same volume sweep at a higher speed to compensate.

But why are compressors used for low temp applications physically larger if they don't require the same sweep as a compressor used for higher temp applications. Also is an EPR fitted to a mid temp evap unit on a multi temp system on a single condenser to prevent the common suction line from going prematurely high and increasing the back pressure, rather than it being pumped down when the low temp evap unit is calling for cooling.

The best solution is to retrofit the existing R404A system to R134A as pecmsg suggested.

I've done this numerous times with zero issues. Often the existing TXV body will work with just a powerhead swap. Other than that, a new filter-drier, a reset of pressure controls, connect the fans for 24/7 operation and configure the timeclock for air defrost is all that needs to be done.

If you have a headmaster condenser flooding control though, it'll need to be changed out...or clipped if you're lucky.

Originally Posted by chilliwilly

Thank's

Just remind me and please do correct me if I'm wrong but for low temp - low load and lower suction pressure, mid temp - higher load and suction pressure, high temp - even higher load and even higher suction pressure. If I'm correct which application would require a higher volume sweep resulting with a higher mass flow?

That would depend on the refrigerant but lets say R-404A

At 0°F evaporator your suction pressure will bee around 20#'s.
At 30°F evaporator your suction pressure will be around 60#'s

Can the pump handle that volume of refrigerant? Need the #'s to say!

Thank's

Just remind me and please do correct me if I'm wrong but for low temp - low load and lower suction pressure, mid temp - higher load and suction pressure, high temp - even higher load and even higher suction pressure. If I'm correct which application would require a higher volume sweep resulting with a higher mass flow?

Originally Posted by chilliwilly

Thank's for the swift replies.

The gas is 404 a but unfortunately I don't have the pot details, I am going to change the xv head and sensor and leave the valve in situ and just check and adjust the superheat if necessary. Maybe even change the injector if its too big.

I'm more concerned about the theoretical low side suction pressure that might be caused by the compressor that was serving a low temp application, and it will make the superheat difficult to adjust.

I'm going to call to site this afternoon to check a few things over such as the pot nomenclature

I've changed the refrigerant in situations like yours from 404 too 134 and had great results. Unfortunately your doubling the load on the freezer pump so that may not be an option.

The compressor performance curve will let us know the max suction pressure allowed!

Thank's for the swift replies.

The gas is 404 a but unfortunately I don't have the pot details, I am going to change the xv head and sensor and leave the valve in situ and just check and adjust the superheat if necessary. Maybe even change the injector if its too big.

I'm more concerned about the theoretical low side suction pressure that might be caused by the compressor that was serving a low temp application, and it will make the superheat difficult to adjust.

I'm going to call to site this afternoon to check a few things over such as the pot nomenclature

What refrigerant was in the low temp system?
MOP Valve in use?

Giving us the comp #'s would help!