How to Design Refrigeration Systems

[US Iceman]

I see posts every so often that ask this question. As a result I thought it might be worthwhile to post some basic information on the subject.

First, let's assume we want to keep something cold. That's the reason for refrigeration in the first place.

Now let's say we want to maintain the temperature in the enclosed space at 0°F (-17.7°C). To produce this temperature the evaporator needs to be colder. So, for this example we select the evaporator temperature to be -10°F (-23.3°C). -10°-0°F = 10° of temperature difference to be used for selecting the evaporator capacity (or -23.3°C--17.7°C = 5.5° temperature difference in SI units).

Since we know there will be some pressure loss in the suction line we have to select the suction line diameter so that the pressure loss does not exceed what was used in designing the system. Let's say the pressure loss is 2 psi (0.14 bar). Since refrigerant flows from higher pressure to lower pressure the pressure at the evaporator is higher than the suction pressure at the compressor. What the compressor will do is produce the cooling capacity at the suction pressure.

Then the compressor increases the refrigerant pressure from the suction pressure up to the discharge pressure. Remember, the refrigerant flows from higher pressure to lower pressure. This means the discharge pressure will be greater than the condensing pressure.

Therefore if the ambient air temperature is 95°F (35°C) the condensing temperature has to be greater than the ambient temperature so that the refrigerant can actually condense. Using an example for the condenser; if the ambient air temperature is 95°F (or 35°C) the condenser capacity could be selected for 115°F (46.1°C). This indicates the condenser heat rejection capacity is based on temperature difference (condensing temperature minus ambient air temperature) of 115°F-95°F = 20° (or 46.1°C-35°C = 11.1°).

If the condensing temperature is 115° (or 46.1°C) then the condensing pressure is equal to the saturated refrigerant temperature (which is the condensing temperature). The discharge pressure of the compressor is greater than the condensing pressure as mentioned before.

What this means is; when you select the compressor capacity the operating conditions you use are; the suction pressure and discharge pressure.

Why????

Because the evaporator pressure occurs at the evaporator and the condensing pressure occurs at the condenser. The pressures at the compressor suction and discharge valves are different than the pressures at the evaporator and condenser.

I may have missed some small point in this explanation, but let's see where the discussion goes...;)

I'm sure someone will come here and point out any flaws.:D

[GXMPLX]

… 95°F (35°C) the condensing temperature has to be greater than the ambient temperature so that the refrigerant can actually condense. Using an example for the condenser; if the ambient air temperature is 35°F (or 35°C) …

Flaw number one you typed the numbers correctly the first time, not the second.

When you test compressors and want to publish data you use saturated suction temperature at compressor inlet pressure and saturated discharge temperature at compressor discharge pressure.

With all your impeccable calculations you did exactly that, calculate saturated suction temperature and saturated discharge temperature.

This plus a little luck, means compressor will either match or be close to match requirements. And you very well know that being close enough is what this business is all about!

…Or maybe I don’t understand the question... My fault!

[US Iceman]

...want to publish data you use saturated suction temperature at compressor inlet pressure and saturated discharge temperature at compressor discharge pressure.


No argument from me, but you have to careful when reading the performance ratings.

Sometimes the operating conditions are stated as "Evaporating Temperature & Condensing Temperature"! What they should be listed as is: Saturated Suction Temperature (SST) and Saturated Discharge Temperature" (SCT). Then the ratings would be closer to the truth (except for any liquid subcooling and high gas return temperatures:D).

Somehow I knew you would be the first to respond to this post....;) I'm glad you did not disappoint me.

[GXMPLX]

I suspect that catalogs are prepared by marketing staff.

If you are lucky they checked the numbers! The rest why bother?, write some statement subcooling liability!

Don't get me started with subcooling I don't think the compressor should account for it, unless it has a particular desing characteristic like intermediate pressure port.

So why use it to select the compressor, leave it as part of a safety factor because errors estimating load can be large. Even if they are not large the load usually increases with time.

[abet_meneses]

iceman,

good points sir.but we should do first the load calculation based on a particular commodities on various application.a manual calculation based on experience is very reliable in designing.software seems to be helpful it depends on the person using it.

[abet_meneses]

iceman,

good points sir.but we should do first the load calculation based on a particular commodities on various application.a manual calculation based on experience is very reliable in designing.software seems to be helpful it depends on the person using it.:)

[US Iceman]

Don't get me started with subcooling I don't think the compressor should account for it, unless it has a particular design characteristic like intermediate pressure port.


I feel the same way. Subcooling is all too often used to artificially increase the compressor ratings. People see this and think, GREAT! Let's use these values because they look sooooo good. That's just not right in my opinion.

The ratings are derived on the basis of an agreement to use ARI methods for stating component performance. ARI has this for all of the components. A somewhat common rating method was intended to furnish a baseline for all performance ratings.

[US Iceman]

...we should do first the load calculation based on a particular commodities on various application.


You are correct. The load calculations are the first task to accomplish. If we did not know the loads, how could we design a refrigeration system to meet that load, right?

My original thought was to start a discussion on how to select components to ensure the system works properly (and meets the cooling load requirement).

[GXMPLX]

I feel the same way. Subcooling is all too often used to artificially increase the compressor ratings. People see this and think, GREAT! Let's use these values because they look sooooo good. That's just not right in my opinion.

The ratings are derived on the basis of an agreement to use ARI methods for stating component performance. ARI has this for all of the components. A somewhat common rating method was intended to furnish a baseline for all performance ratings.

I know you will understand me in the following and be warned I am by far more extremist than you are:

The industry has taken very long to acknowledge that compressor capacity and system capacity are not synonymous, and many still don’t completely do it yet.

The compressor pumps refrigerant, it gets what it can from suction and discharges what it can at different conditions.

Because you are not (or should not) be near saturation you need four variables to specify working conditions (2 suction, 2 discharge) and probably this is why somebody decided to use saturated temperatures. This was intelligent but it oversimplifies things… and causes endless discussions.

As you well say evaporating and condensing conditions are given by external information like room temperature and cooling fluid conditions.

Almost all you read from drawing a cycle in a diagram is system related, like subcooling, useful (or not) superheat, discharge line losses, suction losses, condensing pressure, etc.

(I like to look at compressor with mid pressure port a little differently because they can improve system performance over other compressors that don’t have it).

The enthalpy difference in suction side that has always been a system property that was from the beginning assigned to the compressor because of the way you measure compressor capacity in the lab, that is measuring power input of a heater.

What is compressor information then?

The main is THE one you cannot read out of ANY type of diagram that is, the mass flow. (I don’t like this name). And is this important in your system!

The second is given input conditions what are the output conditions of the refrigerant. This has never been too important as system information except it is very important for the RELIABILITY of the compressor and other system components.

The input conditions are partly given by system but of course once the compressor is chosen you have to recalculate everything (iterate until the solution converges, like us geeks like to say it!)

Is subcooling something that should appear in compressor ratings? Definitely no!

Does subcooling affect compressor capacity. (move it world!) the answer is definitely NO!

The only thing subcooling does is move the compressor input conditions to a point with less superheat and lower specific volume it and yes it pumps more. No contradiction! The proof is that you can design another system that has NO subcooling and exactly the same mass flow, input and discharge conditions! Therefore it is not subcooling that made the compressor perform better it was simply moving input conditions.

Ok, let's leave it here now and wait for reactions. Then I'll hear my wife's "I told you!".

[US Iceman]

Good points in all....



...and be warned I am by far more extremist than you are.


Ohhh, I don't think so.:D

[Josip]

Hi, :)

...nice discussion ....

.... agree with you both .... subcooling is something we can use if we need (but never for compressor capacity calculation) - using economizer we can increase some % of refrigeration capacity for free (not exactly-I know, there is nothing for free ;))...., but for compressor capacity I think it is better to use swept volume and then check do we have what we want at different inlet and outlet conditions ....

Best regards, Josip :)

[Peter_1]

The only thing subcooling does is move the compressor input conditions to a point with less superheat and lower specific volume it and yes it pumps more. No contradiction! The proof is that you can design another system that has NO subcooling and exactly the same mass flow, input and discharge conditions! Therefore it is not subcooling that made the compressor perform better it was simply moving input conditions.

GXMPLX, - couldn't you find an easier nickname ;) - why will the input point of the compressor move to a point with less superheat? The superheat is in my opinion solely controlled by the TEV which will try to regulate a fixed, preset SH, whatever the SC may be.

I see it this way: the SC will indeed not increase compressor performance but evaporator capacity will increase due to increased enthalpy of the fed liquid to the evaporator. This increased evaporator capacity will increase slightly evaporating pressure which in turn will result in an increased COP of the compressor because compression ratio decreases. So compressor will pump slightly more.

[US Iceman]

:off topic:

I was afraid of this...we are getting into a discussion on what the compressor capacity is based on.

Compressors pump gas, they don't really produce cooling. But, because we like to have easy labels to use for capacity we call this Tons of Refrigeration (TR) or kw (cooling).

You can change the compressor capacity by increasing the subcooling (let's just say the subcooling is supplied by something to keep the discussion easy to understand).

I'll present an example in IP units (because it's easier for me;)).

CFM = cubic feet per minute (this is the compressor displacement that is useful; not the total displacement)

Gas density = pounds per cubic foot


CFM / gas density = mass flow (pounds per minute)

mass flow X (hg-hf) = Btu per minute (cooling capacity provided by enthalpy difference and mass flow)

hg = vapor enthalpy (Btu per pound)
hf = liquid enthalpy (Btu per pound)

Therefore, if the CFM and mass flow remain constant the only thing that can change the compressor capacity (assuming constant RPM) is an increase or decrease in the enthalpy difference.

So...if we assume the vapor enthalpy is constant then the only variable is the liquid enthalpy. If you raise the enthalpy of the liquid (warmer liquid) the capacity decreases because the enthalpy difference has decreased.

If you lower the liquid enthalpy (colder liquid) the enthalpy difference increase, which translates into a net increase in refrigeration capacity.

It's not magic...it's thermodynamics!:)

[GXMPLX]

GXMPLX, - couldn't you find an easier nickname -

Yeah too bad, Mxyzptlk was already taken!



why will the input point of the compressor move to a point with less superheat? The superheat is in my opinion solely controlled by the TEV which will try to regulate a fixed, preset SH, whatever the SC may be.

I did not mention any type of expansion device I suppose it is clear for you this happens with other types of expansion device. My wife warned me not to go into the TEV arena. I’ve only had few rounds with US_Iceman and noticed I have to be in good shape so he doesn’t whack me again! Soon I’ll post something like he did on this subject.



I see it this way: the SC will indeed not increase compressor performance but evaporator capacity will increase due to increased enthalpy of the fed liquid to the evaporator. This increased evaporator capacity will increase slightly evaporating pressure which in turn will result in an increased COP of the compressor because compression ratio decreases. So compressor will pump slightly more.

Read what you wrote here! In summary you agree with:
If you get higher suction pressure you get lower specific volume.

At least the suction conditions of the compressor changed, and that an increased mass flow took place.

So subcooling had the effect of moving input conditions (the main theme of my post).

The only thing you don’t agree is me saying you get lower superheat at compressor suction. I was thinking in general.

Well as always my wife comes to help telling me that things ain’t quite as simple and . Let’s read it again (and mess a little with US_Iceman’s main theme, sorry!):



I see it this way: the SC will indeed not increase compressor performance but evaporator capacity will increase due to increased enthalpy of the fed liquid to the evaporator.
Well no, here you mean enthalpy difference at the evaporator increases, but subcooling makes enthalpy of the liquid fed decrease.

This is an effect, but the most important effect of subcooling is that you don’t produce so much flash gas and the refrigerant fed into the evaporator has lower quality.



This increased evaporator capacity will increase slightly evaporating pressure which in turn will result in an increased COP of the compressor because compression ratio decreases. So compressor will pump slightly more.

Yes, the lower quality refrigerant increases heat transfer coefficients in the evaporator and you pick up more heat so the heat picked by evaporator does not remain constant in this process, if it did you would get lower suction pressure because the valve would tend to close for it would sense too low superheat! (So at least in transient conditions my statement was right! Lower superheat at the evaporator outlet means lower at the compressor too)

OK so the valve reacted, it kept a constant superheat, you are picking more heat and suction pressure went up.

If suction pressure went up the expansion valve bulb must be sensing a higher temperature than before in order to keep superheat constant.

Suppose the remaining superheat at the compressor suction was due to a pressure drop between the bulb of theTEV and the compressor. This pressure drop is now higher because you have more refrigerant mass flow.

There are two effects now and the compressor suction will have a lower superheat if the increase in pressure drop … nah, my wife's right! let’s leave this for TEV post and not upset US_Iceman too much besides it is very hard for me to explain it without drawing a chart.

[GXMPLX]

Again US_Iceman I knew it but I posted mine then read the one above it.

I did some effort not to change the subject... did not succeed though...sssooorrryyy!

[david2008]

What this means is; when you select the compressor capacity the operating conditions you use are; the suction pressure and discharge pressure.



Good post!

When we select compressor, will they provide suction pressure and discharge pressure? If yes, we just choose the least power. If no, How could we take the mass flow rate into consideration?

[US Iceman]

...When we select compressor, will they provide suction pressure and discharge pressure?


My first question is: who are they?

What I'm trying to do with this discussion is to break down the assumptions people use.

Evaporating pressure minus suction line pressure loss = suction pressure at the compressor (or, you could say; Suction pressure + suction line pressure loss = evaporating pressure).

Discharge pressure minus discharge line pressure loss = condensing pressure at condenser (or, you could say; Condensing pressure + discharge line pressure loss = discharge pressure).

The point I'm trying to make with this is; suction and evaporating are totally different as is condensing and discharge. People tend to assume these are the same, i.e., suction = evaporating & discharge = condensing. Nothing could be farther from the truth!

What the manufacturer has printed in their catalogs may not be entirely correct and proper. I know of one lawsuit where the compressor manufacturer was held partially liable because of this simple point.

The engineers did not notice the difference until someone else told them.:rolleyes:

Most people will not worry about mass flow rate because it's confusing to talk about and understand. That's why we use Tons or kW (cooling). This is much simpler because; a Ton is a Ton (or a kW is a kW).

Don't assume someone else knows. You have to understand it so that someone else does not make you responsible for their mistakes.

Whatever capacity you have for the evaporator must be equal to the capacity of the compressor. This is done by making sure you include the pressure losses in the piping.

Remember...any pressure loss in piping can be equated to a corresponding loss in saturation temperature. That's why we can them pressure-temperature charts.;)

[GXMPLX]

When the books mention that there are four main components in a refrigeration system (compressor,condenser,expansion device, evaporator) people tend to take this out of context, the books are talking about an IDEAL refrigeration system.

REAL refrigeration systems have SEVEN MAIN components (compressor, discharge line, condenser, liquid line, expansion device, evaporator, suction line).

Why these and not others is because you can build a system without any other components but you cannot without any of these, in a real world not the lab.

So when you are planning a system you should take into account what happens in all these components and that is a main point on US_Iceman’s very good post.

[US Iceman]

When someone selects a component (evaporator, compressor, expansion device, or a condenser) this equipment performance is based on the manufacturers catalogs or software. All of the different components have to be selected to work together to provide the desired results, i.e., a refrigeration system that works properly.

Unfortunately, if someone does not include the pressure losses in the piping (what I call the fifth element of basic refrigeration systems) the performance that was printed in the catalogs or provided by the software will not be developed. As a result, the owner does not receive the full value of his investment....

[monkey spanners]

Eighth component - refrigerant

MS

[GXMPLX]

Eighth component - refrigerant

MS

Right, note taken!

[US Iceman]

If we call it a refrigeration system, wouldn't the use of a refrigerant be implied? :p

Likewise, if we did not include the piping we would only have refrigeration parts! :D

[wambat]

My first question is: who are they?

What I'm trying to do with this discussion is to break down the assumptions people use.

Evaporating pressure minus suction line pressure loss = suction pressure at the compressor (or, you could say; Suction pressure + suction line pressure loss = evaporating pressure).

Discharge pressure minus discharge line pressure loss = condensing pressure at condenser (or, you could say; Condensing pressure + discharge line pressure loss = discharge pressure).

The point I'm trying to make with this is; suction and evaporating are totally different as is condensing and discharge. People tend to assume these are the same, i.e., suction = evaporating & discharge = condensing. Nothing could be farther from the truth!

What the manufacturer has printed in their catalogs may not be entirely correct and proper. I know of one lawsuit where the compressor manufacturer was held partially liable because of this simple point.

The engineers did not notice the difference until someone else told them.:rolleyes:

Most people will not worry about mass flow rate because it's confusing to talk about and understand. That's why we use Tons or kW (cooling). This is much simpler because; a Ton is a Ton (or a kW is a kW).

Don't assume someone else knows. You have to understand it so that someone else does not make you responsible for their mistakes.

Whatever capacity you have for the evaporator must be equal to the capacity of the compressor. This is done by making sure you include the pressure losses in the piping.

Remember...any pressure loss in piping can be equated to a corresponding loss in saturation temperature. That's why we can them pressure-temperature charts.;)

It seems to me that if one is learning this profession that all of the above is considered when one is properly schooled on the subject anything less is a diservice to the student, trade, and the customer. I believe if a person is truly dedicated to this profession he/she should have a passion to be the best one can be at all times and take whatever measures/sacrifices is required to always keep abrest of the technology advancements. The most amazing thing about this profession is the scope of knowledge required to be professicient in all its aspects and the satisfaction of knowing the you are doing a great service for the general welfare of your society

[GXMPLX]

If we call it a refrigeration system, wouldn't the use of a refrigerant be implied? :p

No, several refrigeration systems do not use refrigerant, Peltier, Magnetic, Acoustic.

In that line of thought wouldn't pressure drops be implied?


Likewise, if we did not include the piping we would only have refrigeration parts! :D

You are rignt but people tend to forget they should consider them, so why not better point them out from the beginning?

[david2008]

My first question is: who are they?

What I'm trying to do with this discussion is to break down the assumptions people use.

Evaporating pressure minus suction line pressure loss = suction pressure at the compressor (or, you could say; Suction pressure + suction line pressure loss = evaporating pressure).

Discharge pressure minus discharge line pressure loss = condensing pressure at condenser (or, you could say; Condensing pressure + discharge line pressure loss = discharge pressure).

The point I'm trying to make with this is; suction and evaporating are totally different as is condensing and discharge. People tend to assume these are the same, i.e., suction = evaporating & discharge = condensing. Nothing could be farther from the truth!

What the manufacturer has printed in their catalogs may not be entirely correct and proper. I know of one lawsuit where the compressor manufacturer was held partially liable because of this simple point.

The engineers did not notice the difference until someone else told them.:rolleyes:

Most people will not worry about mass flow rate because it's confusing to talk about and understand. That's why we use Tons or kW (cooling). This is much simpler because; a Ton is a Ton (or a kW is a kW).

Don't assume someone else knows. You have to understand it so that someone else does not make you responsible for their mistakes.

Whatever capacity you have for the evaporator must be equal to the capacity of the compressor. This is done by making sure you include the pressure losses in the piping.

Remember...any pressure loss in piping can be equated to a corresponding loss in saturation temperature. That's why we can them pressure-temperature charts.;)



Thanks for reply.:)

1.Let’s assume we want to select a proper compressor.Here is an example of performance ratings.

Evap temp is 20F, cond temp is 100F. (From compressor manufacturer)

Compressor I: 1385BtuH, 225W, 20Lb/Hr (high temperature compressor)
Compressor II: 2235BtuH, 346W, 30Lb/Hr (high temperature compressor)
Compressor III: 2235BtuH, 325W, 30Lb/Hr (medium temperature compressor)

I am wondering these different compressors match what kind of systems? If my system is also designed on Evap temp of 20F and cond temp of 100F. How to compare between high temperature and medium temperature compressor?

2.In your post, you use tons as the unit. What is the convert factor between ton and btu/h? (I remember on a paper, people say a 10HP compressor is a 10 ton system. I don’t understand. ) When to design cond, the capacity of cond = evap capacity + compressor heat. How to calculate compressor heat. If we choose compressor III in my first paragraph, is the compressor heat is 325W? :eek:

Thank you.

[Peter_1]

I believe if a person is truly dedicated to this profession he/she should have a passion to be the best one can be at all times and take whatever measures/sacrifices is required to always keep abrest of the technology advancements. The most amazing thing about this profession is the scope of knowledge required to be professicient in all its aspects and the satisfaction of knowing the you are doing a great service for the general welfare of your society
Perhaps not direct related to USIceman's post but how true this is!!! 100% agree with you.

[Peter_1]

F
When you test compressors and want to publish data you use saturated suction temperature at compressor inlet pressure and saturated discharge temperature at compressor discharge pressure.

With all your impeccable calculations you did exactly that, calculate saturated suction temperature and saturated discharge temperature.

I teach it this way in school: I give a certain evaporator capacity and long and far too small suction lines - hypothetical case - and let them chose then a compressor.
In fact a little bit the same way.

Related to this, I still find the equilibrium diagram - don't know if this is the right English expression - usefully when learning to understand what happens when a parameter changes.
But I have to confess that I need to reread this specific section every year before I give these classes.

[Peter_1]

Sometimes the operating conditions are stated as "Evaporating Temperature & Condensing Temperature"! What they should be listed as is: Saturated Suction Temperature (SST) and Saturated Discharge Temperature" (SCT).
US Iceman, you call these SST and SCT, you're speaking here about compressor conditions I suppose.
I call it in classes 'extra' superheated gas at the inlet and somewhat higher discharge pressures due to HP lines.

Can we speak of SST seen by the compressor because for me SST is exactly on the 100% line of the log/p and these aren't the entrance conditions on the compressor?

All by all, if lines are proper selected, much difference will not occur in smaller applications.
But it's good that someone sees that there's a difference which leads to a somehow smaller system capacity when not taken in account.

[Peter_1]

If you lower the liquid enthalpy (colder liquid) the enthalpy difference increase, which translates into a net increase in refrigeration capacity.

GXMPLX says in post 9 Does subcooling affect compressor capacity. (move it world!) the answer is definitely NO! but you states the opposite, at least the way I'm reading it.
In my opinion, it will increase as described some posts higher of me.

[nevgee]

Equilibrium diagram = balance graph to show the balance point (point of equilibrium) of the evaporator, compressor and condenser.
We would encourage our younger engineers to draw a balance graph after they had selected the evap / compressor /condenser configuration, as an aid to visualising how the system capacities would converge to a balance point at predetermine conditions. This could then be compared to the design load before progressing onto completing the final design and component selections.
I think now people are to eager to use software to do the selection and balancing without seeing graphicaly what is happening in the system as a whole.

[US Iceman]

GXMPLX says in post 9 Does subcooling affect compressor capacity. (move it world!) the answer is definitely NO! but you states the opposite, at least the way I'm reading it.
In my opinion, it will increase as described some posts higher of me.

This is where the explanation gets confusing.

If subcooling is available and used to overcome the pressure losses (horizontal or vertical piping) then the subcooling does not increase the compressor capacity. The subcooling is being used to keep the refrigerant in the liquid line at 100% liquid. If you get flash gas in the liquid line then the compressor capacity (cooling capability) will decrease.

If the subcooling available is greater than the above, then the additional subcooling helps to increase the compressor cooling capability. This is due to the greater enthalpy difference available with the remaining subcooling.

[US Iceman]

US Iceman, you call these SST and SCT, you're speaking here about compressor conditions I suppose.


That's correct Peter. The terms SST and SCT as I use them mean the following:

SST = the saturation temperature of the suction pressure at the compressor. It does not include any superheat (evaporator or suction line). It is only the equivalent saturation temperature that takes into account the pressure losses in the suction line. Same logic applies to the high side of the system.



I call it in classes 'extra' superheated gas at the inlet and somewhat higher discharge pressures due to HP lines.


I think we are both saying the same thing. You have superheat from the direct expansion evaporator and then the gas picks up more superheat in the suction line. So we have evaporator superheat and suction line superheat. The total of these equal the total superheat at the compressor.

I think we agree on the basics. There are pressures and both temperatures that can affect the compressor rating.



Can we speak of SST seen by the compressor because for me SST is exactly on the 100% line of the log/p and these aren't the entrance conditions on the compressor?


I believe you are thinking about this as the isotherm line (the purely horizontal saturation line) as the saturation temperature, which is correct. When I sat saturation temperature at the suction I mean the equivalent temperature of the gas or liquid at some pressure. So, on the log/p diagram; when you move to the right that is the saturation temperature for that pressure. If you show the pressure drop moving down at the dew point curve the lower pressure also has a saturation temperature. Does that make sense?



All by all, if lines are proper selected, much difference will not occur in smaller applications.
But it's good that someone sees that there's a difference which leads to a somehow smaller system capacity when not taken in account.

Absolutely! On bigger systems the losses may be huge and put the person in an embarrassing situation they have to explain. Therefore, if the person uses the basics on all systems (large or small) they will not have this problem.

[US Iceman]

david2008,

I spent 40 minutes working on some explanations for your post and lost the whole darn post somehow. I will have to recreate it when I have more time.

Stay tuned!

[Peter_1]

This is where the explanation gets confusing.

If subcooling is available and used to overcome the pressure losses (horizontal or vertical piping) then the subcooling does not increase the compressor capacity. The subcooling is being used to keep the refrigerant in the liquid line at 100% liquid. If you get flash gas in the liquid line then the compressor capacity (cooling capability) will decrease.

If the subcooling available is greater than the above, then the additional subcooling helps to increase the compressor cooling capability. This is due to the greater enthalpy difference available with the remaining subcooling.

Totally agree and understood.

[david2008]

david2008,

I spent 40 minutes working on some explanations for your post and lost the whole darn post somehow. I will have to recreate it when I have more time.

Stay tuned!

I appreciate. Thank you.;)

[US Iceman]

OK, I'm going to take another stab at this post. It might not be as good as the original, but then no one saw the first one because I somehow deleted or lost it. Computer literate, right?:o




Evap temp is 20F, cond temp is 100F. (From compressor manufacturer)

Compressor I: 1385BtuH, 225W, 20Lb/Hr (high temperature compressor)
Compressor II: 2235BtuH, 346W, 30Lb/Hr (high temperature compressor)
Compressor III: 2235BtuH, 325W, 30Lb/Hr (medium temperature compressor)

I am wondering these different compressors match what kind of systems?





You essentially have three basic operating ranges defined:
Low temp
Medium temp
High temp
What this means is: the manufacturers have matched their compressor displacements (CFM or liters/second) with various motor sizes to fit many different ranges of applications.

You should not be comparing the compressor capacities based solely on the operating range. Take the first two for example. If your evaporator you want to use on this system has a capacity of 1300 Btu/h (@ 20°F evaporating temperature) then compressor 1 has the closest capacity match. The compressor actually has a slightly greater capacity than required. Therefore, if we factor in the pressure loss of the suction line (say the pressure loss is equivalent to a 1° temperature loss. That puts your SST at the compressor = 19°F (not 20°F). So, if you calculate the compressor performance at 19°F you will see it's capacity decrease slightly. In effect, it will be pretty close to the evaporator capacity @ 20°F. Do you see what I just did. The compressor capacity is based on the equivalent temperature at the compressor due to the pressure losses.

If you use one of the other compressors the capacity will be greater than you need and the suction pressure will run lower. This happens because you have more compressor displacement than what the evaporator will create due to boiling.

This is only a start, however you need to see the logic in doing this.






If my system is also designed on Evap temp of 20F and cond temp of 100F. How to compare between high temperature and medium temperature compressor?


See above...



2.In your post, you use tons as the unit. What is the convert factor between ton and btu/h?


1 Ton = 12,000 Btu/h
1 Ton = 200 Btu/minute

The basis for the term Ton of Refrigeration (usually just called Tons or Ton, or sometimes labeled as TR) is 288,000 Btu per 24 hours. This comes from the old days when we used ice for refrigeration. Originally the definition was: 1 Ton = 288,000 Btu per 24 hours (1 day) to melt one Ton of ice (2,000 pounds) into 32°F water.



I remember on a paper, people say a 10HP compressor is a 10 ton system. I don’t understand.


I think using this type of terminology or logic is nonsense. This rule-of-thumb only applies to systems operating at air conditioning temperatures and is based on several assumptions that are only valid at one point. They do not apply to anything else and are confusing to use if you don't know this.

A medium temperature compressor will have an average compressor displacement for the size of motor fitted to the compressor.

A low temperature compressor will have a larger displacement and a relatively smaller motor.

A high temperature compressor will have a smaller displacement with a larger motor provided.

This is due to the volume of gas generated at low, medium or high suction temperatures to produce 1 Ton of Refrigeration.

So... what you may see is:

A lot temperature compressor may have a power/capacity ratio (usually called BHP/TR or kW/TR). BHP = Brake Horsepower which is the equivalent power input shown in the catalogs. Sometimes the manufacturers will show watts or kW for the power input.

A medium temperature compressor will have a lower ratio, while a high temperature compressor will have an even lower power/capacity ratio.




When to design cond, the capacity of cond = evap capacity + compressor heat. How to calculate compressor heat. If we choose compressor III in my first paragraph, is the compressor heat is 325W?


Total Heat of Rejection = compressor capacity + power input

Both the capacity and power input must be expressed in the same units (Btu/h or equivalent), but you want to put them in the same units that the condenser is rated in (usually MBH; 1 MBH = 1,000 Btu/h)

Some people use the term Tons for heat rejection. Again, this is misleading at it is based on certain assumptions that may not apply to any other application.

For the time being, I'll stop here. I have already lost one and not tempted to try for two.

[david2008]

OK, I'm going to take anot...;....
For the time being, I'll stop here. I have already lost one and not tempted to try for two.

Thank you for typing the answer second time. ;)Recently I learned a lot from you and other friends in RE. I hope I can answer other's question like you reply me soon.:cool: RE is a nice forums.

David

[US Iceman]

I don't mind helping, but remember... when you have an opportunity to help someone in this business...you should.

As the title to the movie said...pay it forward!:cool:

[GXMPLX]

US_Iceman did a very good job with this post, let’s give it a new push!

Several design considerations are at play when the manufacturer chooses an application, US_Iceman mentioned the main one, note that in all these the torque the motor develops is not constant.



A high temperature compressor will have a smaller displacement with a larger motor provided.


It is hard to generalize so I’ll stick to reciprocating (possibly semi hermetic) compressors.

The main design objective here is high efficiency because you spend more energy and is the most widespread application but also you get the highest refrigerant flow and density therefore:


Compressors should minimize internal pressure drops including the valve plate because of high density and high flow. If you compare a valve plate you’ll see bigger (or more) bores.
Cooling the motor is easier in semi hermetics so you can divert internal flow not superheat it so much.
Thermal losses are of less concern because temperature differences are less.
Thermal stress is of less concern for the same reasons.
Gas re-expansion is of less concern due to low compression ratio.
Lubrication temperature is less critical.
Mechanical loads in general are less due to low compression ratio.


This also means some drawbacks if you go to the extremes of the range:


High suction and low condensing pressures can really overload the motor.
Quickly looses efficiency and capacity if suction pressure is low due to high gas re-expansion.




A medium temperature compressor will have an average compressor displacement for the size of motor fitted to the compressor.

In general these compressors extend the high temperature range improving on volumetric efficiency.

The problem is most manufacturers fit large motors in order to extend their range, sacrificing efficiency in high temperatures to have better efficiencies in the lower range.

If you compare capacity of a medium temperature compressor is generally less than a high temperature compressor from a given temperature and higher and has more capacity from that point down. This is the crossover point that would make you decide if you have to choose.



A low temperature compressor will have a larger displacement and a relatively smaller motor.
Well here the list is almost the opposite of above:


Internal pressure drop is of little concern due to low density low flow of refrigerant.
Cooling the motor is a MAYOR concern because you may not get enough refrigerant to do it well under all working conditions. You may need external cooling at some conditions.
Thermal losses are of big concern because temperature differences between suction and discharge are very high. So you try to separate both flows as much as you can.
Thermal stress is of concern you get thicker metal.
Gas re-expansion is a MAYOR concern due to high compression ratio.
Lubrication temperatures are critical and need external cooling.
Mechanical loads in general are high due to high compression ratio.


These compressors used at high suction can be more efficient than medium range compressor but that is because the motor can be nearly overloaded.
More capacity in the high end of its temperature range than medium range compressors due to higher volumetric efficiencies so there is also a crossover point here.

This also means some drawbacks if you go to the extremes of the range:


High suction can overload the motor and produce motor protector trips.
High discharge temperatures can limit the application severely.
Reliability is a MAYOR concern.


There are many more (of course) feel free to add to the list!

As I have posted elsewhere you should use P-H diagrams to look closely on where the problems of your application may be, densities, discharge temperatures, superheat, subcooling.

Use T-S diagrams if you want to study or compare efficiencies because it has to do with the area of the cycle.

Use equilibrium diagrams to see how the system will behave under varying external conditions in ambient and the cold room.

US_Iceman will gladly reply to all your posts on this!

[US Iceman]

US_Iceman will gladly reply to all your posts on this!


It is so nice to see someone dig a hole and then give the shovel to someone else.

Or perhaps, you might consider not opening this...

http://skugg.files.wordpress.com/2007/06/can-of-worms.jpg

Having said that...:D

Let's see where this goes....

[GXMPLX]

... but I was learning to use the list label BB code!

It is always easy to be generous with other people's time and money!

Did you recover from the slingjuice?

[david2008]

US_Iceman will gladly reply to all your posts on this!

:cool:you dig a good hole!

[david2008]

I am thinking about a question. It may be proper to reply this topic.

If we design a system. Every machine is the right size. System run good now. what will happen if we oversize the evaporator or condenser?

[GXMPLX]

:cool:you dig a good hole!

And full of worms!

I like it when you dig them out of the ground it means it (the ground) is alive yet!

[GXMPLX]

I am thinking about a question. It may be proper to reply this topic.

If we design a system. Every machine is the right size. System run good now. what will happen if we oversize the evaporator or condenser?

It depends on how much you oversize and if you took this into account

Oversized evaps (in general) increase superheat, improve efficiency but too much could produce distribution problems, hunting, low system capacity.

Oversized condensers improve system efficiency (if taken into account when sizing TEV) but if the pressure differential is too small produce same problems as above.

[david2008]

It depends on how much you oversize and if you took this into account

Oversized evaps (in general) increase superheat, improve efficiency but too much could produce distribution problems, hunting, low system capacity.

Oversized condensers improve system efficiency (if taken into account when sizing TEV) but if the pressure differential is too small produce same problems as above.

I understand. Thank you.:-)

[tinman59]

:eek:nice some one who realy knows the bizz im realy impressed

[US Iceman]

Let's give this a twist and see what happens...:)



Over-sized evaps (in general) increase superheat, improve efficiency but too much could produce distribution problems, hunting, low system capacity.

Over-sized condensers improve system efficiency (if taken into account when sizing TEV) but if the pressure differential is too small produce same problems as above.


All true.... if this is for a DX system.;)

On a properly designed liquid overfeed system you will not see these effects. The system efficiency continues to increase as the heat exchanger areas increase. The cost does also.:D

[GXMPLX]

nice some one who realy knows the bizz im realy impressed

Yeah US_Iceman has worms of knowledge!

Let's give this a twist and see what happens...

All true.... it this is for a DX system.

On a properly designed liquid overfeed system you will not see these effects. The system efficiency continues to increase as the heat exchanger areas increase. The cost does also.

You are right! But you know we can never generalize to cover every crazy little system out there, I was only talking about 99.998% of them!

How’s that for a twist!

[US Iceman]

...you know we can never generalize to cover every crazy little system out there.


You generalized this but then did not say it applied to DX systems.

Twist returned.:D

[GXMPLX]

Ouch! So much twisting hurts, better let’s go back and continue the straight road!


…

You essentially have three basic operating ranges defined:

Low temp
Medium temp
High temp

…


Different manufactures define different values for these ranges and people don’t understand that they overlap.

What would be your range of temperatures?


-40F to 0F
-10F to 32F
10F to 50F

[GXMPLX]

Sorry posted twice.

[US Iceman]

I would say your temperature ranges are reasonably valid. Those are as good as anything I might suggest.

[GXMPLX]

Do you ever check your visitor messages?

[ayat]

Thank you for explenation of tons.
it is interesting for me.

[Jemal]

Hi everyone, and thanks in advance, for the first time in my life I will work in transformer oil cooling system. I don’t know how to select perfect capacity and right machineries. The transformers capacity is 9250 KVA made by Tamini with seven ton of oil in each unit and they are used in iron alloys company. Oil temperature is now reaching 85ºc , it should drop 20ºc to stay in 65ºc. ambient temperature is 50ºc Jubail (Saudi Arabia)

[vivek_padole]

how to design air handling unit?

[vivek_padole]

how tod esign the air handling unit

How to do the base frame,filter frame calculations,how to calculate coil size area

[GXMPLX]

Hi everyone, and thanks in advance, for the first time in my life I will work in transformer oil cooling system. I don’t know how to select perfect capacity and right machineries. The transformers capacity is 9250 KVA made by Tamini with seven ton of oil in each unit and they are used in iron alloys company. Oil temperature is now reaching 85ºc , it should drop 20ºc to stay in 65ºc. ambient temperature is 50ºc Jubail (Saudi Arabia)



Interesting problem you should try a new post.
All the ideas I can think of maybe are not new to you.


First check it has its full charge of oil and the right kind of oil.
It should be kept clean of dust at least from the heat exchanger HE.
Protect it from the sun. Protect the supporting metal structure with a highly reflective paint.
If there is no wind you could try installing a sun powered fan (crazy he?)
If there is wind check the temperature you could be getting hotter than ambient air if it is heated by near structures. In this case you need to protect it from hot air or enhance the HE surface. If you get no prevailing wind you still get air from local convective currents if they are hot you need to cut them for example installing metal sheets on the ground to break the current air pattern.
If the ambient air temp is ok you might have an overloaded transformer or not the right HE for the conditios and this should be checked with the manufacturer.

[adrianshanov]

If I wanted to design my own evaporator for 0F deg, where do I start from? How do I select the tube length, diameter, and fins?

[GXMPLX]

I would start by reading one of the many books that come with EES!

You can read a good list in the reference section of this paper:
http://www.me.gatech.edu/energy/susan/papers/HEFAT.pdf

[sethukani]

hi how do desine -60 refrigeration

[sethukani]

hi how do desine -80 domasticrefrigerator
evaporator tube size & lenth ,which gas is correct which type of condesor is best please sent mail

[sethukani]

[hi how to de sign -30c to -60c
[/quote]
cooler