Effect of Hot Gas and Flash gas on Evaporators

US Iceman suggested I start this thread, as we were getting off topic on another.
I think these two subjects are related, although different.

By US Iceman;

<TABLE cellSpacing=0 cellPadding=0 width="100%" border=0><TBODY><TR><TD class=smallfont vAlign=top width="100%" bgColor=#ffffff>The first question I would have relates to the loss of the flash gas, which is used to help with liquid distribution through the evaporator circuits. Ohhh, this sounds like fun!:D </TD><TD style="BACKGROUND-IMAGE: url(/forums/images/misc/quotes/quot-right-bg.gif)" width=10></TD></TR><TR><TD vAlign=top width=10>http://www.refrigeration-engineer.co...t-bot-left.gif</TD></TR></TBODY></TABLE>
And from myself;

I have always wondered about-the effect on evaps of introducing hot gas. (As with hot gas bypass)

I have not given much thought to the first, but have always wondered about the effect of the second.

OK, if we say we are concerned with hot gas bypass in this thread, let's start with some questions.

Also, for the time being let's keep this discussion limited to DX evaporators, OK?

First question: What exactly does the hot gas provide when used as a hot gas bypass?

Second question: Where should the hot gas be introduced?

What exactly does the hot gas provide when used as a hot gas bypass? A way to keep the suction pressure up during light loads, when you do not want the Compressor to cycle, or you need to keep a constant Suction pressure.

Where should the hot gas be introduced? Into the Suction Line is one option. Unfortunately this requires you to De-superheat the Suction Line in most cases. Gets to be a bit complicated. The other option is downstream of the TX Valve, or through a side port on the distributer.

Sporlan lists a third option, but I do not recall it at the moment.

Quote:

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Originally Posted by NH3LVR

A way to keep the suction pressure up during light loads, when you do not want the Compressor to cycle, or you need to keep a constant Suction pressure.

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If the compressor capacity is greater than the volume flow rate of gas out of the evaporator, then the suction pressure decreases. Suppose the compressor has capacity reduction in 25% increments. When you unload down to 25%, the suction pressure will continue to decrease if the load continues to reduce.

This could lead to the compressor shutting off on low-pressure, and then re-starting quickly, which contributes to short cycling and loss of evaporator control.

The hot gas injected into the evaporator simply replaces the gas formed by liquid refrigerant boiling in the evaporator due to a heat load.

As the evaporator pressure begins to decrease, the hot gas bypass regulator senses the drop in pressure and begins to open.

I prefer the hot gas injection downstream of the TXV through a side port distributor myself. This helps to mix the liquid and hot gas into a uniform mixture. Then since the TXV is controlling the evaporator superheat the TXV can provide better control.

If you use the hot gas into the suction line with the liquid injection for desuperheating you have two TXV's to deal with. Why complicate things?

The difference between the two methods is: where do you mix the liquid refrigerant and hot gas?

If this is done in the evaporator, I feel this is much better and provides improved results.

If this is done in the suction line, you are dependent on the mixing occurring in the suction line before the compressor.:eek:

Hot gas bypass should be selected for the lowest step of compressor capacity reduction to ensure a constant evaporating pressure when very close control of the refrigeration system is required.

If the compressor has no capacity regulation, then this is a very wasteful means to control the system.

Quote:

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Originally Posted by US Iceman

If the compressor has no capacity regulation, then this is a very wasteful means to control the system.

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Agreed, however in some applications, like Eutectic Plates, it is essential in order to completely freeze the plates at the end of the cycle. I have used this myself. Systems with large pressure drops benefit from this approach, avoiding rapid cycling.
We briefly considered this last week on a ***** system that only had half of the planned evaporators installed. We decided as a temporary measure to remove some suction valves instead, effectively unloading the compressor.

Agreed. I have also used this procedure for environmental rooms requiring constant temperature control.

One of the more interesting applications was a 3000 HP 2-stage system where we had to run the (2) boosters at 100% load for two weeks before the evaporators were connected to the system.

So, for this two week period we had the twp boosters @ 100% and the two hi-stage compressors @ 60%, with absolutely no cooling being done. It was all hot gas and liquid injection. Now that was a fun one!:D

The similarity of this and the removal of flash gas are similar. If you remove the gas (hot gas or flash gas) from the evaporator, the liquid has a more difficult time uniformly wetting the coils.

In the case of flash gas, the evaporators and distributors are designed to use this to help distribute the liquid in the evaporators.

Removing the flash gas improves the cycle efficiency, but impacts on the evaporator performance. That's why I think the project being considered for a masters thesis is so appealing to me.:cool:

Quote:

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Originally Posted by US Iceman

The similarity of this and the removal of flash gas are similar. If you remove the gas (hot gas or flash gas) from the evaporator, the liquid has a more difficult time uniformly wetting the coils.

In the case of flash gas, the evaporators and distributors are designed to use this to help distribute the liquid in the evaporators.

Removing the flash gas improves the cycle efficiency, but impacts on the evaporator performance. :cool:

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I once was beginning a discussion with a Design Engineer about the negative effects of extreme subcooling on TX Systems. Unfortunately we were interrupted and he never got to make his points. Could this have been what he was referring to?

What would this cause-underfeeding or overfeeding, or just poor performance?

Very well could be. If the liquid is subcooled too close to the evaporating temperature the liquid refrigerant may just lay there as it flows through the coil circuits, Upon leaving the coil, the superheat would be zero so the TXV would just shut down. This could lead to an unstable operation resulting in hunting.

This is only my opinion.

A similar effect occurs on overfeed evaporators also if the liquid fed to the coil is a lot colder than the evaporating temperature.

This usually occurs when a very low liquid feed temperature from one vessel is used to feed an evaporator that is controlled at a higher pressure (to achieve a warmer coil temperature) with a back-pressure regulator.

Hi US iceman and NH3LVR

Really it is very interesting thread but i have many comments so plz bear with me:

Hot gas bypass

It is used in AC applications as one method for cooling capacity control. The main idea of this method is to bypass some discharge gas from the compressor at the condensing pressure to the evaporator after TXV or to the suction line. Of course the preferable one is to bypass the gas after TXV and I will say why afterwards.

The main idea of this control method is modulate the system capacity to match the drop in cooling load without excessive cycling in case of the thermostat used in huge system work under 20% to 25% cooling load reduction.

The capacity modulation in hot gas bypass is accomplished by two ways:

1- Reduce the volume of refrigerant entering the evaporator and so the system cooling capacity decreases and also the evaporating pressure is increased as a result of a smaller refrigerant volume rate. This is done of the type of hot gas bypass to the compressor suction line, the main shortcomings of this kind is the compressor is most likely to be overheated as a result introducing hot discharge gas to the suction line and so liquid injection should be also introduced to the suction line wisely without risk of the compressor flooding and complicated design of two TXV as US iceman said. So it is not preferred or widely used in AC application.

2- Reduce the refrigerating effect by introducing hot pressurized gas via two means increase the evaporating pressure and by increase the refrigerant temperature entering the evaporator so both account for an increase in refrigerant entering enthalpy.

Quote:

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Originally Posted by US Iceman

Hot Gas Bypass should be selected for the lowest step of Compressor capacity reduction to ensure a constant evaporating pressure when very close Control of the refrigeration system is required.

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I disagree little bit with this statement, Hot Gas Bypass should be selected for the lowest step of Compressor capacity reduction to ensure the compressor is protected form a rapid cycling either by low-pressure cut out or on/off cycling in low cooling load.

Because the hot gas bypass does not provide a constant evaporating pressure it only increases it to some what extent to protract the system operation. In contrary, the system which involves hot gas bypass control should have condenser pressure control (head pressure control) because the condenser only carries the load of compressor motor power and slight cooling load so the condensing pressure is drastically dropped. Furthermore, the condenser receives partial refrigerant volume capacity as a result of the bypassing effect. So the discharge pressure is also dropped and in turn the hot gas bypass can not maintain the evaporating to the same level with the design.

Cheers

Flash gas

Also I disagree with this paragraph. Firstly, the evaporator cooling capacity depends on two stands: the refrigerant mass flow rate and refrigerating effect. Simply

Qe = mr * (1- vapor dryness faction)*refrigerant latent heat at certain evaporating temperature

The more liquid introducing to the evaporator (means with less dryness faction) the more increase in refrigerating effect and capacity in the same conditions. This is the main idea of separation the flash gas from the refrigerant mixture (gas &liquid) before entering the evaporator.

The refrigerant distribution does not rely on the existence or not existence of the flash gas it mainly depends on the evaporator and distributor proper design.

The only pitfall of the flash gas releasing is to the oil return to the compressor is to some what extent is badly affected as a result of deceleration of the mixture flowing inside the evaporator because the mixture has lower density than that for liquid and in turn the refrigerant velocity is higher and can scrub the oil from clinging on the tube inner surface and carry it over to the compressor. This is talent is missed in case of pure liquid feeding the evaporator and so the oil separator is considered as a must in this case typically in case of liquid overfeed evaporator.

In a short, removing the flash gas improves the cycle efficiency and enhances the evaporator performance.

The problems can be mentioned with removing the flash gas can be stated as:
1- Complicated design by increase number of compressor in low and high stages
2- The second HP stage compressor is likely to be liquid flooded unless there is baffles or strainer used in the flash intercooler chamber
3- The two stage compression need intercooler system which sometimes water-cooled cooler and another additional component.

Quote:

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Originally Posted by US Iceman

Very well could be. If the liquid is subcooled too close to the evaporating temperature the liquid refrigerant may just lay there as it flows through the coil circuits, Upon leaving the coil, the superheat would be zero so the TXV would just shut down. This could lead to an unstable operation resulting in hunting.

This is only my opinion.

A similar effect occurs on overfeed evaporators also if the liquid fed to the coil is a lot colder than the evaporating temperature.

This usually occurs when a very low liquid feed temperature from one vessel is used to feed an evaporator that is controlled at a higher pressure (to achieve a warmer coil temperature) with a back-pressure regulator.

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Subcooling

Firstly the more liquid subcooling the more enhancements cycle efficiency but on expense of oil return problem, the explanation is:


Cooling capacity = refrigerant mass flow rate * refrigerating effect


For a certain cooling capacity, the more increase in subcooling effect the more increase in refrigerating effect and in turn to a smaller refrigerant charge and a smaller compressor size and power.

However, for the same evaporator surface area the smaller refrigerant mass flow rate means the smaller refrigerant velocity and the same problem of oil carry over in the evaporator so the compromise should be done here.

The subcooling has no authority on the superheat degree because the TXV decrease the pressure and temperature (whatever the value of those) to the required evaporating temperature and pressure. And the superheat degree is confined by the evaporating air temperature and inlet air temperature and the evaporator design itself.

In the liquid overfeed systems there is another issue which is :

The refrigerant mass flow is very big due to the circuiting refrigerant is 4 to 5 the design value to ensure fully wetted evaporator surface area plus the refrigerant enters the evaporator pure liquid. Accordingly the required heat to convert the liquid to vapor is hefty so all the cooling load is used for producing refrigerant mixture or in gaseous form with zero superheat.

The back-pressure regulator is used for two purposes:

1- In Multiple evaporators connected in parallel to ensure a constant evaporating pressure for each one and prevents the instability of the refrigerant flowing.
2- In a single evaporator to prevent the evaporator frosting or in case of chiller to prevent the water freezing.

cheers