Freezer efficiency questions

[M_Q]

I have a freezer running at (say) 0 deg. F with the compressor and condenser
some distance away (about 80 ft.). As usual, the liquid line is not insulated and
the suction line is. The refrigerant is R-409a, which has a heat of vaporization
of about 95 BTU/pound, a specific heat of about 0.29 and 0.17 BTU/pound/deg. F
for liquid and vapor, respectively.

The liquid refrigerant leaves the condenser at (say) 90F, looses (say) 10F through the
uninsulated liquid line and enters the evaporator at 80F where for each pound of
refrigerant that evaporates, 95 BTU is absorbed. Of that, 23.2 BTU (80*.29) is used
to cool the liquid refrigerant and the remaining 71.8 BTU is used to maintain the
freezer at 0F.

Am I mostly correct so far?

The vaporized refrigerant leaves the evaporator at 0F (I am assuming the evaporator
is an efficient heat exchanger), and despite the efforts of the insulation on the
suction line, removes heat from the atmosphere and finally gets
back to the compressor at (say) 60F.

Question 1: Is it true that I am using almost 25% of my cooling on the specific heat
of the refrigerant?

Now, suppose I remove the insulation from that vapor return line, put the liquid and
suction lines in good thermal contact, and put the insulation back around the whole
mess so that I have a nice long heat exchanger. So the liquid will enter the evaporator
cooler, permitting more of the heat of vaporization to cool my freezer, and the suction
line will have a warmer average temperature so I won't be cooling the atmosphere
as much.

Let me assume that the liquid is cooled to 20F before entering the evaporator
(because of the counter flow in our newly created heat exchanger, this should
be possible?). Then, of that 95 BTU heat of vaporization for each pound,
only 5.8 BTU are used in dealing with the specific heat of the liquid refrigerant
and 89 BTU can be used to cool my freezer.

Has my freezer just gotten 24% more efficient?
What is wrong with this?
Why is this not done when there is a large temperature difference condenser and
evaporator temperature, as is the case with a freezer?

[Gary]

In cap tube freezers, it is not uncommon to solder the cap tube to the suction line and this is sufficient heat exchange.

[Bones]

lots of freezer coils (depending on size) do have a heat exchanger located inside the room this is done by the same principal gary is explaining exept the liquid line passes through the suction line before heading to the tx valve in the coil.

the more you can subcool your liquid before it gets to your metering device the more effective it will work this is correct.

the exact gains made can be seen if you were to plot a ph diagram of the current cycle, then redo another once you have made your heat exchanger.

[Gary]

Now, suppose I remove the insulation from that vapor return line, put the liquid and
suction lines in good thermal contact, and put the insulation back around the whole
mess so that I have a nice long heat exchanger.

If you are going to place suction line and liquid line "in good thermal contact", solder them together to increase thermal contact and avoid electrolysis.

[Andy]

If you are going to place suction line and liquid line "in good thermal contact", solder them together to increase thermal contact and avoid electrolysis.


I went to a freezer once that the condensing unit had been replaced on and two engineers couldn't get down in temperature, low suction, high head:eek:

The unit was above the false celing and had a condenser fitted for the low temp range only. So I determined that the liquid line was a discharge line so the onlt thing to do was change the unit or help it get down to design conditions.

So I stripped off the armaflex and cable tied the 3/8" liquid to the 7/8" suction and rearmaflexed the lines.

The room came down no problem:) if it was up to me I would have changed the unit, but the commercial service manager was happy the customer was happy, so there was no problem:eek:

Kind Regards Andy:)

[Peter_1]

Is there anyhow a saving?

The heat of the liquid line is transported to the suction, so the mass flow and especially the temperature of the suction gasses will increase.
What you save on one is lost again on the other side.
Just throwing in this thoughts.

You will have no flas gas at the metering device, that's for sure.

[Gary]

Is there anyhow a saving?

The heat of the liquid line is transported to the suction, so the mass flow and especially the temperature of the suction gasses will increase.
What you save on one is lost again on the other side.
Just throwing in this thoughts.

You will have no flas gas at the metering device, that's for sure.

There is only an energy savings if the evaporator consequently absorbs more heat due to its increased efficiency (increased active area). The heat exchanger, as you say, is a trade off.

[Peter_1]

Agree Gary, so you must mount the bulb of the TEV after the HE which seems forbidden by all the HE manufacturers.
You then haven't evaporator SH anymore.

[Gary]

Agree Gary, so you must mount the bulb of the TEV after the HE which seems forbidden by all the HE manufacturers.
You then haven't evaporator SH anymore.

This can be gotten around by using two HE, with the bulb mounted in between them.