Recirculations ammonia evaps

[Friobernal]

Hi

Although my last post was about the same subject.. I want to insist:

Playing with programs of selection of flooded ammonia evaps, i have seen that the difference between 2 and 4 recirculations for the capacity of the evap is minimum (almost 1 kW) for P = 46 kW,
but selecting a pump recirculations are quite important of course.

I also have seen that the refrigerant mass flow and velocity are almost double.

Which is the thermodynamical reason for this little difference in the capacity of de evaporator?

Thanks all

[Prof Sporlan]

Which is the thermodynamical reason for this little difference in the capacity of de evaporator?

The 2002 ASHRAE Refrigeration Handbook has an informative chapter on overfeed systems (Chapter 1). It notes that the ideal circulation rate for an evaporator is a function of: heat load, pipe diameter, circuit length, refrigerant, distribution, evap temperature, and whether the coil is top or bottom fed.

For R-717, it recommends a minimum circulation rate of 2 to 4 for bottom fed coils, and 6 to 7 for top fed coils.

Developing a model to predict recirc evaporator coil performance would, no doubt, be a significant undertaking.

[Andy]

Hi Friobernal,
recirc rates anything above 1 will give you a fully wetted evaporator. Increasing the circ rate/mass flow may cause a little better heat transfer by turbulance, especially in an over sized evaporator, little capacity is gained. When designing pumped plant the extra circulated refrigerant is to ensure a fully wetted evaporator, even when all evaporators on a system are requiring refrigerant.
Another reason for higher that required circ rates are to defeat the pressure held back by any evaporator pressure regulators fitted to increase evaporation in some chills running at a higher temperature that the main chill on a system. Evaporation control is also sometimes required to ensure that the product is not dehydrated, or the personell standing in a chilled area are not exposed to very low air off temperature.
Kind Regards. Andy

[Dan]

?

Playing with programs of selection of flooded ammonia evaps, i have seen that the difference between 2 and 4 recirculations for the capacity of the evap is minimum (almost 1 kW) for P = 46 kW,

Hi all.

If I understand Friobernal, I am thinking there are charts out there that assume no difference in the temperature difference between the coil and the room temperature.

I also have seen that the refrigerant mass flow and velocity are almost double.

I am a bit out of my league here, but is there any chance that when you increase the recirculation rate that you can arrive at a significant increase in the required net refrigeration effect of the refrigerant side of things simply because you have increased the effective temperature difference between the coil and the air?

Is Friobernal telling us that he sees what a chart predicts, and has applied it only to find that his load increases dramatically?

[Friobernal]

Thanks all

Hi Andy and Prof, you always enlighten me in ammonia world

I´ve got now several ideas:
1. Increasing recirculations(i.e. from 2 - 4), we increase the mass flow and the velocity (inside of evaporator)
2. This is good for remove oil and get a better heat transmission
3. We also ensure that the inside surface of the tubes is wet
4. As the flow we are looking for in a flooded evaporator is anular
the k is not going to change (a lot) for an increase of mass flow of ammonia due is evaporating and the pressure drop is no going to be very high (changing this)

What would happen if instead of 4 we will increase till 10 recirculations?

Bye

[Prof Sporlan]

I am a bit out of my league here, but is there any chance that when you increase the recirculation rate that you can arrive at a significant increase in the required net refrigeration effect of the refrigerant side of things

Increasing the circulation rate simply increases the heat exchange effectiveness of the coil. The inside surfaces of the tubes in the coil, in effect, get "wetter" with increased circulation rate.

There is a point of diminishing returns, however. At some point, the additional energy needed to increase circulation rate will outweigh the additional heat exchanger effectiveness obtained.

What would happen if instead of 4 we will increase till 10 recirculations?

You would end up buying a much bigger pump....