M_Q
13-03-2007, 06:53 PM
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?
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?