Capillary tubes

[marc5180]

I saw a question on this forum a while back and no one answered it. I've looked on google but dont get a definitive answer and i don't know.

To provide greater cooling capacity capillary tube restrictors;

a) should be shorter with a bigger bore
b) would increase flow automatically due to higher condensor pressure
c) should be long but with a bigger bore
d) should be connected directly to the evap inlet.

I would say either a or c...........

Ok i say "C" .

Is that right?

[nicolacozma]

I saw a question on this forum a while back and no one answered it. I've looked on google but dont get a definitive answer and i don't know.

To provide greater cooling capacity capillary tube restrictors;

a) should be shorter with a bigger bore
b) would increase flow automatically due to higher condensor pressure
c) should be long but with a bigger bore
d) should be connected directly to the evap inlet.

I would say either a or c...........

Ok i say "C" .

Is that right?

Increasing of cooling capacity is not related only with capillary tube modification. Greater cooling capacity means a greater flow rate of refrigerant through the compressor, and as a consequence the flow rate of the liquid through the capillary tube have to be greater.
Only a) solution can do this.

The c) solution wil give you, roughly, the same pressure drop across the capillary tube and this means the same flow rate of liquid.
If you increase the flow rate of liquid using a) method then you have to be carefull because you will have a wet compression with all the consequences.

[wambat]

It’s critical that the designed length and diameter of the capillary tube are appropriate to the system’s designed evaporator and condensing pressures, and must be equal to the compressor’s pumping capacity at these same design conditions. So if they are (designed for the above conditions) then no other cap tube flow rate will work at the designed conditions.

[Chef]

To provide greater cooling capacity capillary tube restrictors;

a) should be shorter with a bigger bore
b) would increase flow automatically due to higher condensor pressure
c) should be long but with a bigger bore
d) should be connected directly to the evap inlet.

I would say either a or c...........

Ok i say "C" .

Is that right?

Assuming the only thing that is to be changed is the tube dimesions then to get greater cooling.....

We must add another item to the list:-
e) should be longer with the same bore.

Typically when a tube system reaches its steady state running condition the entry to the tube is about 20% gas and 80% liquid. So there is only 80% of the refrigerant available for cooling effect.

By increasing the length of the tube the quality at the entrance will move from 0.2 towards 0 and the system flow rate will be the same and the system pressures will be the same. Now all the refrigerant is liquid and so increased cooling effect. This may seem extremely wierd but when both ends of the capillary tube are in the two phase zone strange things happen.

The downside is when you try to start the system from ambient, it will have very high discharge pressures and will probably trip out on compressor overload. So it is possile if the startup current can be handled.

Chef

[nike123]

I saw a question on this forum a while back and no one answered it. I've looked on google but dont get a definitive answer and i don't know.

To provide greater cooling capacity capillary tube restrictors;

a) should be shorter with a bigger bore
b) would increase flow automatically due to higher condensor pressure
c) should be long but with a bigger bore
d) should be connected directly to the evap inlet.

I would say either a or c...........

Ok i say "C" .

Is that right?

I would say

d) should be connected directly to the evap inlet.

Reason is, because all heat picked up in tubes before evaporator inlet is not useful and lowers our true cooling capacity. Imagine spit system with long uninsulated pipes, and it will be clearer.

[marc5180]

Anyone else? That does make sense Nike butthen again increasing the bore also makes sense

[icecube51]

The downside is when you try to start the system from ambient, it will have very high discharge pressures and will probably trip out on compressor overload. So it is possile if the startup current can be handled.

Chef

is it not way they have more than one cappilairy tube.to have a better refrig flow and lower presure at start-ups??

Ice

[marc5180]

My college tutor, thinks it is "C"

[nike123]

My college tutor, thinks it is "C"

Did he gave you reasoning for that choice?

[marc5180]

He says this will increase the flow capacity of the tube but........ I've been reading up and i think that you ( Nike) are right.

I've taken this out of "Principles of refrigeration" by Roy Dossat page 353.

however when the tub does not have enough resistance (tube to small and/or bore to large), the flow capacity of the tube will be greater than the pumping capacity of the compressor at the design conditions, in which case over feeding of the evaporator will result with the danger of possible liquid floodback to the compressor. also, there will be no liquid seal in the condensor at the entrance to the tube and therefore, uncondensed gas will be allowed to enter the tu be along with the liquid obviously the introduction of latent heat into the evaporator in the form of uncondensed gas will have the effect of reducing system capacity.

[nike123]

He says this will increase the flow capacity of the tube but........ I've been reading up and i think that you ( Nike) are right.

I've taken this out of "Principles of refrigeration" by Roy Dossat page 353.

however when the tub does not have enough resistance (tube to small and/or bore to large), the flow capacity of the tube will be greater than the pumping capacity of the compressor at the design conditions, in which case over feeding of the evaporator will result with the danger of possible liquid floodback to the compressor. also, there will be no liquid seal in the condensor at the entrance to the tube and therefore, uncondensed gas will be allowed to enter the tu be along with the liquid obviously the introduction of latent heat into the evaporator in the form of uncondensed gas will have the effect of reducing system capacity.

Then challenge him with Dossat quote!

[marc5180]

I will take the book along with me to show him

[Chef]

I would say

d) should be connected directly to the evap inlet.

Reason is, because all heat picked up in tubes before evaporator inlet is not useful and lowers our true cooling capacity. Imagine spit system with long uninsulated pipes, and it will be clearer.

a) will make it more inefficient b) does not make sense and c) will probably be the same original so I reckon your d) choice is the only one left and agree with the analogy.

Chef