Evaporator too big.

[nuntachai]

To all

Let's suggestion to me , How the system occur ? , when we applide of the biger evaporator than system ( Compressor 25 kw cooling cap , Evap Coil 45 kw cooling cap).

Thank you for all kindness.

[Sandro Baptista]

To all

Let's suggestion to me , How the system occur ? , when we applide of the biger evaporator than system ( Compressor 25 kw cooling cap , Evap Coil 45 kw cooling cap).

Thank you for all kindness.

Hi nuntachi,

Let see if I understand: you want to know what's the behave of a refrigerant plant when we have a compressor and a evaporator with differente capacities, right?

Well, I give you an example:

Suppose you have the following project parameters:

- Air cold store temperature: -25ºC
- Evaporating temperature: -32ºC

Now suppose that we have a compressor that at -34ºC (2 K suction loss, for instead) of pressure suction equivalent saturation temperature regime will give 25 kW.
And you have a aircooler evaporator that will give 45 kW at -25ºC (air temperature) and at -32ºC evaporating temperature.

The system will reach an equilibrium between compressor and evaporator which will result in a refrigerant capacity of the system that will be bigger than 25 kW and less than 45 kW. This happens because:

Less than 45 kW because you can mantain the DT = 7K since you have only 25 kW of compressor capacity;
Bigger than 25 kW because the suction pressure in the compressor will rise (» bigger evaporating temperature) due the fact that the aircooler can give more than 25 kW at a DT less than 7K at -25ºC cold store temperature.
I hope have been clear.

Regards

[Sandro Baptista]

Hi nuntachi,

Let see if I understand: you want to know what's the behave of a refrigerant plant when we have a compressor and a evaporator with differente capacities, right?

Well, I give you an example:

Suppose you have the following project parameters:

- Air cold store temperature: -25ºC
- Evaporating temperature: -32ºC

Now suppose that we have a compressor that at -34ºC (2 K suction loss, for instead) of pressure suction equivalent saturation temperature regime will give 25 kW.
And you have a aircooler evaporator that will give 45 kW at -25ºC (air temperature) and at -32ºC evaporating temperature.


The system will reach an equilibrium between compressor and evaporator which will result in a refrigerant capacity of the system that will be bigger than 25 kW and less than 45 kW. This happens because:

Less than 45 kW because you can mantain the DT = 7K since you have only 25 kW of compressor capacity;
Bigger than 25 kW because the suction pressure in the compressor will rise (» bigger evaporating temperature) due the fact that the aircooler can give more than 25 kW at a DT less than 7K at -25ºC cold store temperature.
I hope have been clear.

Regards


I forget to complete my thought :): I estimate an capacity (in the equilibrium of the system) of maybe about 30 kW.

[nike123]

Compressor capacity will be the same at same evaporation and condensation temperatures because it is determined by refrigerant density and compressor displacement (mass flow).
For compressor it doesn't matter how big is evaporator if evaporation (and condensation) temperatures are not changed.

But fact is that at same cell temperature, bigger evaporator will have smaller TD, and that evaporation temperature will be higher than with smaller evaporator.
That higher evaporation temperature mean that compressor have higher capacity than with lower evaporation temperature achieved with smaller evaporator (because of his higher TD).


But, we need to keep humidity in control, and that is determined by TD. Therefore, it is essential to match evaporator and compressor to achieve targeted humidity in refrigerated space.
Did I complicated or cleared things?:D

[desA]

But fact is that at same cell temperature, bigger evaporator will have smaller TD, and that evaporation temperature will be higher than with smaller evaporator.
That higher evaporation temperature mean that compressor have higher capacity than with lower evaporation temperature achieved with smaller evaporator (because of his higher TD).

But, we need to keep humidity in control, and that is determined by TD. Therefore, it is essential to match evaporator and compressor to achieve targeted humidity in refrigerated space.
Did I complicated or cleared things?:D

I agree. The oversize evaporator, with higher saturation temp, can mean that the compressor has to work much harder. It also then leads to higher compressor discharge temperatures.

For a heat-pump, this becomes a real issue, I've found.

Sometimes (?often?) ex refrigeration/aircon designers merrily oversize the evaporator for a heat-pump - but, this can lead to major system imbalances & reduction in COP off the optimum thermodynamic COP value.

Best to finely match evap/compressor/condenser specifically to your target application.

[nuntachai]

Thank a lot, Sandro
It quite clear.

[nuntachai]

Hi nike123 and thank you for suggestion......
nike123 , when temperature of evaporator higher(higher than smaller evaporator) ,that meaning heat reject at the condenser will be increasing?

[desA]

^ Yes, and then some. The COP will also probably reduce, since the compressor work will, in all likelihood, increase excessively.

[US Iceman]

...that meaning heat reject at the condenser will be increasing?


Yes.

If the larger evaporator is used the compressor suction pressure will be higher. This results in greater cooling capacity for the reasons nike123 provided. With the increase in compressor capacity, the heat rejection also increases. This may require you to provide a larger condenser also!

[desA]

The compressor manufacturer may often have a table, or software program, where compressor/evaporator/condenser duties are matched at a certain set of operating conditions (evaporator Tsat, condenser Tsat).

Copeland & Bitzer provide useful software for this.

As I see it, the heart of the refrigeration process is really the compressor, with the evaporator & condenser sized to fit around this.

[Sandro Baptista]

But, we need to keep humidity in control, and that is determined by TD. Therefore, it is essential to match evaporator and compressor to achieve targeted humidity in refrigerated space.

Taking the example I have done at my first post (Air temperature cold store = -25ºC, Evaporating temperature = -32ºC) if you have a used existing bigger evaporator that you want to install that will low the DT, as have told, so less water vapor of the air will condensing in the cold coil » higher relative humidity. For a freezer cold store, a bigger H.R. 90%, for instead, is positive. Picking again the example I have done the evaporating would increase to about -30ºC » DT = 5K is completely acceptable.

If you don't need to keep low the humidity, having a bigger evaporator doesn't do warm at all, it will guarantee a bigger relative humidity, wich is desired for most applications.

[Sandro Baptista]

I agree. The oversize evaporator, with higher saturation temp, can mean that the compressor has to work much harder. It also then leads to higher compressor discharge temperatures.

For a heat-pump, this becomes a real issue, I've found.

Sometimes (?often?) ex refrigeration/aircon designers merrily oversize the evaporator for a heat-pump - but, this can lead to major system imbalances & reduction in COP off the optimum thermodynamic COP value.

Best to finely match evap/compressor/condenser specifically to your target application.


I don't agree with you.

Keep in mind that if the condenser is selected to have the same condensation temperature parameter of design I can ensure that is not true. Come on...about what you said "that the compressor has to work much harder"we are talking in an increasing of about a few degrees of higher evaporting temperature (maybe 2...3K of higher evaporating temperature). The COP will be higher, the discharge temperature will be lower because the ratio of the absolute pressure will be smaller (p_cond/p_suction) and the compressor will work little bit less time.
Of course that the due the fact that the suction pressure will be a little higher the connection rods, the cranck shaft and piston rods will be little more subject to mechanical tensions but is not relevant. Possible, in global due the fact the discharge temperature is lower the durability of the compressor will be the same.

[nuntachai]

Thank a lot for all of you.

[Magoo]

I may have lost a few details in translation. But get back to basics. Everyone can pontificate to the enth degree. The system has to be balanced.
Big evaps versus smaller compressors is not unusual, the design eng,, may have required a high sensible heat ratio, or high air flow and a low system TD at the evaporator, what ever. Several things have to be addressed and checked.
1/ the TEV matches the compressor duty.
2/ The TEV has [ mop ]
3/ The distributor orifice and distributor tails are matched to TEV performance.
4/ The compressor charactoristics can handle high suction pressures at times.
5/ The condenser duty matches the various conditions of load and ambient conditions.
6/ The TEV superheat readings have to be checked.
All above are essential. TEV sizing, and orifice and tail tube sizing can create havic, TEV hunting , liquid slugging, high suction superheat, lack of system performance.
Until all above are checked and adjusted, the compressor will rev into and early grave.
magoo

[desA]

^ To add to this, use the Magoo Rule for SH setting:

SH = (0.6 to 0.7)*TD

[mbc]

In very simple explain. Evaporator Capacity = 45 KW in what TD :Usually TD = 8 to 10:<O:p</O:p
if we see 45 KW in 10 TD So For 1 TD it is 4.5 KW<O:p</O:p
If your compressor capacity is 25 KW your TD is about 25/4.5<O:p</O:p
Do not forget if your evaporator temperature <O:p</O:p
Is goes up your compressor capacity also goes up Until 2 of them (( comp. and Evap. )) is come equal.<O:p</O:p

Less Td is good for keep humidity in room

[AJS]

^ To add to this, use the Magoo Rule for SH setting:

SH = (0.6 to 0.7)*TD


How is TD calculated in this case??

[desA]

TD = Ta,in - Te,sat

The Kueba site will provide more detail on the computation.

[AJS]

[quote=desA;162537]TD = Ta,in - Te,sat

Okay, thanks! :)

[mbc]

My main from TD for Evaporator (Delta T) = Room Temperature – Evaporator Temperature
<O:p</O:p

[Sandro Baptista]

TD = Ta,in - Te,sat

This will be TD1



Room Temperature – Evaporator Temperature will be TDm


TD1 and TDm could be very different and consequently can give significant difference to the evaporator capacity, specially if the airflow is low and/or the TD is small.

[AJS]

TD = Ta,in - Te,sat

This will be TD1



Room Temperature – Evaporator Temperature will be TDm


TD1 and TDm could be very different and consequently can give significant difference to the evaporator capacity, specially if the airflow is low and/or the TD is small.

So if i'm following you correctly, TD1 = temp air in - saturated evaporator temperature, whereas TDm is?

Temp air in should be the same as room temp right? and evaporator temp would be same as saturated evaporator temperature?

And what is the correct one to use when determining system capacity?
thanks, AJS

[mad fridgie]

So if i'm following you correctly, TD1 = temp air in - saturated evaporator temperature, whereas TDm is?

Temp air in should be the same as room temp right? and evaporator temp would be same as saturated evaporator temperature?

And what is the correct one to use when determining system capacity?
thanks, AJS

TD1 and TDm generally are the same, but under some circumstances they can differ, for example when air is forced through a warm product and returned directly to the evap.
Evap temp is higher than SST because you have suction line pressure drops and non useful superheat.
System Capacity is determined at the compressor (inlet and outlet conditions) Nett refrigeration is based upon conditions entering the TXV and outlet conditions leaving the evap.

[Sandro Baptista]

TD1 and TDm generally are the same, but under some circumstances they can differ, for example when air is forced through a warm product and returned directly to the evap.
Evap temp is higher than SST because you have suction line pressure drops and non useful superheat.
System Capacity is determined at the compressor (inlet and outlet conditions) Nett refrigeration is based upon conditions entering the TXV and outlet conditions leaving the evap.

I' dont agree with the statement TD1 and TDM generally are the same.

room temperature is the (Tair inlet + Tair outlet) / 2. Is a medium value of the air temperature.

For example for a TD1 = 8K that could correspond to a TDM of about 7K or 6 K. It depends of the working conditions and the aircooler design.

[mad fridgie]

I' dont agree with the statement TD1 and TDM generally are the same.

room temperature is the (Tair inlet + Tair outlet) / 2. Is a medium value of the air temperature.

For example for a TD1 = 8K that could correspond to a TDM of about 7K or 6 K. It depends of the working conditions and the aircooler design.
For design purposes you use you design room temperatue as your air on temperature so basically the the same (MOST but not all control there rooms on return air temp. Stat sensor is normally mounted behind the back of the evap coil) I also stated a typical exceptions. Actual room air temperatue varies depending upon where you take the measurement!

[Gary]

As I see it, the bottom line overriding factor in all of this is humidity and that relates directly to the coil leaving air temperature. Higher humidity is not always desireable, in which case a larger coil is going to need less airflow and/or face bypass dampers in order to drop the air off temp to achieve humidity targets.

[lana]

Hi there,

Equipment selection is based on design conditions.

DTe = T(room) - T(evaporation)

Low DTe like 5°C will result in a bigger evaporator and it will be higher relative humidity in the room.

High DTe like 10°C will result in a smaller evaporator and lower relative humidity in the room.

When the evaporator is bigger than this (DTe lower than 5°C) like 2°C then, room temperature and Te will be very close together and superheat will be very small, which in some conditions you will have liquid flood
back.
Compare two conditions :
1- DTe = 8°C , Troom=+2°C then Te=-6°C
2- DTe=2°C , Troom = +2°C then Te=0°C

in case 1 compressor will give its capacity at Te=-6°C and in case 2 compressor gives its capacity at Te= 0°C which is higher than case one.

System will balance itself at the balance point. When you do the balancing calculation then everything is apparent.

Too big evaporator also is not economically reasonable.


Practically DTe varies between 5°C and 10°C.


Cheers