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Most efficient refrigerator

Discussion in 'Home Power and Microgeneration' started by m Ransley, Jan 25, 2004.

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  1. Guest

    I'm questioning that. Have you ever seen a serious study that says a full
    fridge uses less energy than an empty one? How much less? Googling reveals
    this claim on a few web sites (eg Bob Vila's), but it is unsupported, and
    most of the sites that mention it also recommend cleaning the coils below
    the fridge to save energy, even tho (oddly enough) LBL researchers have
    debunked that myth.

    The Maryland Energy Office site more carefully claims that a full fridge
    "retains cool temperatures better... especially during power outages."

    From _flowing_ room air, driven by temp diffs which increase with more
    cold heat exchange surface with high conductance to cold thermal mass.

    Please note the continuously flowing air, vs a one-time slug replacement.
    Do you have any proof of this claim? To me, basic science teaches the reverse.
    Maybe not :)

    Nick
     
  2. Guest

    Some suggest it's too low.
    I was thinking about a thermal chimney with lots of cold thermal mass
    and mass surface and conductance, and no airflow resistance inside.
    Sounds very organized. I'm thinking upright fridge vs freezer, BTW.
    Wire vs glass shelves would facilitate airflow...
    Yes indeedy. "Some time."
    The airflow might exceed the formula I used during this time. We need CFD.
    Sounds good. The extra day would allow the food to reach the fridge temp.
    I am seldom so pleased with our collective search for truth via the net.

    Nick
     
  3. daestrom

    daestrom Guest

    I'd like to see the LBL cite. Can't possibly understand how dirty coils
    that prevent condensation of freon without higher head pressures would *not*
    be an efficiency issue. I've seen units with so many 'dust bunnies' on the
    coils that with no air flow at all and a service call was made because the
    freezer wasn't keeping the ice-cream hard though running continuous.
    Vacumned of the coils, rolled it away from the wall and vacumned the back
    and floor and put it back into place. Unit promptly shut off the compressor
    in 1/2 an hour.
    reverse.

    Your 'basic science' does not teach the reverse. You just think it does. I
    have pointed out that your natural convection flow formula calculation is
    poorly applied here. Have you compared the area of the entire inside
    surface of an 'empty' (three walls, floor and ceiling) with the 'face'
    (equivalent to the back wall in area) of a stocked fridge? The multiple
    surfaces of food stocks are close together, sometimes touching. Those food
    items that aren't touching form channels with a high hydraulic diameter
    resulting in laminar conditions and much lower flow. The increased flow
    resistance, the poorer surface smoothness, the shorter vertical run before
    another food item obstructs/diverts the flow, are just some of the things
    your 'basic science' ignores. To try and use the natural convection formula
    you have for such a surface and call it 'basic science' is a misapplication.
    Without considering these things, you will have overestimated the flow rate
    through the multiple surface 'channels' by a wide margin.

    The 'empty' has one large open area in the middle so the natural convection
    formula is *close* for the three exposed walls. But as I pointed out, this
    is still way off and not representative of actual usage. Who in their right
    mind leaves the door open long enough for steady state flow conditions to
    establish? The 'slug' falling out of an 'empty' happens differently than
    your formula suggests, and is much larger than the 'slug' from a
    well-stocked unit (say between 5 and 10 times larger). If the door is not
    kept open very long, this 'slug' represents a large portion of the total air
    moved (and is smaller for a 'full' unit).
    Maybe you should just do a study of your own and get back to us?

    daestrom
     
  4. Guest

    Would you have any evidence for this article of faith?
    http://enews.lbl.gov/Science-Articles/Archive/energy-myths3.html and
    http://enduse.lbl.gov/Info/LBNL-45862.pdf

    And the enormous condensation heat transfer coefficient.
    That's one opinion :)
    Agreed. As you say, it seems to UNderestimate airflow.
    Less, IMO, so it's a less-efficient heat exchanger.
    Oh dear. Got any numbers?
    I disagree.
    I disagree.
    It seems to me that the "steady state" has LESS flow than the transient case.
    This reinforces my argument.
    Both of us seem to have some technical understanding of this situation, but
    neither of us have definitive data. Why on earth should anyone believe you?

    Nick
     
  5. daestrom

    daestrom Guest

    These studies say the change in efficiency is not measurable. Fine, but
    they don't detail how much dirt or any test conditions. But I have serviced
    more than a few refrigerators that were running all the time and not keeping
    cold. Probably 25% of them were just because the coils were caked with
    pet-hair, small trash paper and 'dust-bunnies'. I *still* maintain that if
    the unit is running continuously to try and keep cold it must be less
    efficient than one that cycles off/on as designed.

    Maybe their idea of 'dirty coils' is not as severe as what some people think
    of as 'dirty coils'. I'm talking about when you pull the grill off the
    front bottom of the unit and all you see is a 'wall' of hair/dirt. And when
    you roll the unit out, the casters leave tracks. The pet-hair and 'gunk'
    roll out the back of the unit as you pull it away from the wall.
    Underestimate for the 'empty' unit and *OVERestimate for a full unit. Don't
    you see that this would mean the flow for a 'empty' unit is higher than the
    flow for a 'full' unit. And this would skew the energy usage towards the
    nearly 'empty' unit (assuming you bother to open the door, since it doesn't
    have much in it).
    Obviously you would. But to think the natural convection formula for a
    smooth vertical plate with free access flow from the bottom, applies to side
    access flow of several 'jagged' flow channels isn't science. There are some
    assumptions and simplifications in that formula and its application that you
    are ignoring.
    Got any numbers (right back at ya)???
    Shorter, narrower channels, with only one side open will have less
    'penetration depth' of the flow. The air only goes into the unit from the
    open door a few inches. Not the full depth of the compartment. With an
    'empty' unit, it has a much wider channel and the flow stream can actually
    go further back into the unit from the door opening.

    But again, this steady-state flow doesn't tell the whole picture. Probably
    for the average upright fridge/freezer user, steady-state is seldom reached
    except maybe on grocery day when it's time to restock the unit. But then
    the human activity disrupts the flow patterns, so even then it doesn't tell
    us much.
    case.

    YES. The 'slug' flow from a 'full' unit *might* be twice the steady state,
    but the 'slug' from an empty unit could be 10 times the flow of the 'empty'
    unit. Different free volumes, different slug flows.
    No, it actually refutes it. Not because the 'slug' transient flow is higher
    than your steady-state formula, but because it is very different between the
    two. It is *much* higher for a 'empty' unit because there is a much larger
    'slug'. The free volume of the full unit is likely 10% or less of the empty
    unit. So the 'slug' for an empty unit is 10 times the volume of the full
    unit. If the door is only open for a brief time, this 'slug' is a major
    contributor and the *size* of it is significant.

    If the 'slug' were the *only* flow, the 'full' unit would only have about
    10% of the flow of the 'empty'.
    Right back at you, why should anyone believe you?

    But since you asked....

    I model heat-transfer and fluid flow systems with engineering calculations
    for a living, been doing it for 18 years. Mixed gases, steam flows, a
    variety of boiling regimes from nucleate pool to film boiling and dryout.
    Before that, actually worked commercial A/C&R as well as steam plant.
    Before that, 11 years in submarine nuclear power program.

    You quote a formula from ASHRAE (I assume that's where you got this one, you
    get much of your material there). How about some other, more detailed
    "first principles" texts, like.....

    "Thermodynamics", Kenneth Wark
    "Engineering Thermodyamics with Applications", David Burghardt
    "Principles of Heat Transfer", Frank Kreith
    "Engineering Thermodynamics", Burghardt & Harbach
    "Modern Refrigeration and Air Conditioning", Goodheart & Wilcox
    "Fluid Mechanics with Engineering Applications", Daugherty & Franzini

    You slap together a few lines of quick-basic code once in a while and share
    that with the group. Very nice of you and we appreciate it. I
    write/maintain over 100,000 lines of FORTRAN, C, & C++ code to simulate the
    entire steam cycle of a power plant with auxilaries, right down to
    calculating water level in feed-water heater drain tanks and steam-jet air
    ejector mixed gas flows from the main condenser. Next month I'm starting a
    new project to model the HVAC for a turbine building. Customer wants not
    only normal HVAC, but responses for steam break in any room containing steam
    piping, and smoke response for possible fires in rooms containing
    lubricating oil piping or electrical switchgear (including responses of
    cardox systems). Results must include temperature, smoke concentrations,
    steam/vapor concentrations, radioactivity and CO2 and O2 concentrations for
    each room.

    There is some really nice code for modeling HT&FF available from the feds.
    Look for RELAP/RETRAN. Even the manuals for it (available on line) are
    pretty good references. Shows calculational models for a lot of 'neat'
    things like this. Takes a while to setup though, not the sort of trivial
    thing one might do in an afternoon while it's snowing @ 3"/hour outside.

    I think my opinion is every bit as valid as yours.

    daestrom
     
  6. Guest

    They were just summaries of studies. You might look at the studies cited.
    Sounds plausible to me.
    Or the cleaning is not so important as thought to be. A lot of things seem
    significant in principle, but pale with a few numbers. People sometimes
    suggest running a building with micro-hydro electric power from rainwater
    flowing through downspouts, or attaching a 4 watt timer to a water heater.
    I don't think so. It seems to me that the full unit would have more cold
    surface to cool air that flows down through it, so the air would emerge
    cooler, and thus flow faster.
    No. I don't see that.

    I wonder what we can agree on. Suppose the 2'x3'x5' empty unit has 62 ft^2
    of interior walls, and the full one has 5 wire shelves with 24 vertical
    3" diameter x 10" tall 1-liter soda bottles on a 6" grid on each shelf.
    Surrounded by 3" of air, the bottles would introduce minimal flow resistance,
    but they would add 90 ft^2 of cold surface with no insulation behind it,
    giving the full fridge 152 vs 62 ft^2 of cold surface, about 2.5X more...

    Maybe. If you pack your fridge like a box full of bricks...

    I'd estimate that a 1/2" gap would provide minimal airflow resistance.
    This chimney (vs plate) formula might be accurate to an order of magnitude.
    Maybe that's all we need to answer this question.
    With what channel width?
    If the room is 70 F and the fridge is 36, how much free channel area is
    needed to make the air velocity near the back of the fridge at least 90%
    of the air velocity near the front, 2' away?
    It seems to me that the larger initial flows point to full fridges
    using more energy.
    Hey, we may have agreed on something :)
    The empty unit above has a volume of 30 ft^3... 120 1-liter bottles occupy
    about 4 ft^3, so the full fridge would contain about 26 ft^3 of air. But
    these are both very small compared to the volume of air that may flow through
    the fridge when the door is open, since 16.6x2x3/2sqrt(5(70-36)) = 650 cfm.

    30/26 = 10?
    I disagree.
    I have data. This morning my kitchen was 59 F with 40% RH. I opened
    the door of my GE TBX18TAzAERWH fridge (about 5 years old) and used my
    recently calibrated Raytek Raynger ST IR thermometer to measure the temp
    of a spot on the inside wall and the temp of a pickle jar on a shelf:

    Time wall temp pickle jar temp
    (sec) (F) (F)

    0 40.0 39.0
    15 41.5 39.0
    30 42.0 39.0
    45 43.5 39.0
    60 44.0 39.5
    75 45.0 39.5
    90 46.0 39.5
    105 46.0 39.5
    120 46.0 39.5

    An hour before, I put a small mirror inside the fridge. When the door was
    open, I saw no condensation on the mirror. The wall (vs pickle jar) temp
    would increase a lot faster with condensation, no?
    Great. You are an expert :) You can actually answer these questions...
    No thanks. But looking again at the ASHRAE HOF, I see a formula (McAdams,
    1954) for heat transfer by condensation from steam on the outside of vertical
    cylinders: h = 4000/L^0.25dT^0.33, eg 8618 Btu/h-ft^2-F for L = 1' and
    dT = 10 F. The HOF continues (somewhat cryptically):

    Condensation heat transfer rates reduce drastically if one or more
    non-condensible gases are present in the condensing vapor/gas mixture.
    The decrease in the heat transfer coefficient is approximately linear with
    the weight fraction of the noncondensable gas present... In a steam chest
    with 2.89% air by volume, Othmer (1929) found the heat transfer coefficient
    dropped from about 2000 to 600 Btu/h-ft^2-F.

    They go on to describe various methods to estimate this drop, including
    a general method by Colburn and Hougen (1934), but, alas, they are all
    beyond my expertise. I'd also like to know how air interferes with LiCl
    water vapor absorption.

    Pages 14.13 and 14.14 of Rhosenow, et al's incredibly expensive 1484 page
    Handbook of Heat Transfer (McGraw Hill, 3rd edition, 1998) talk about
    non-condensibles even more cryptically:

    Minkowycz and Sparrow [41] solved this problem under free convection
    conditions using a similarity transformation... Chin et al. [43] modeled
    both free and forced convection and solved the complete two-phase boundary
    layer equations using a finite control volume method with an adaptive grid.
    Figure 14.10 shows their results for a steam-air mixture... The serious
    deterioration in heat transfer under quiescent conditions (using the
    Nusselt, pure vapor case) is evident... A very small concentration of air
    of 0.1 percent decreases the heat transfer by about 32 percent...

    They go on to suggest a general calculation method based on Reynolds, Prandtl,
    Nusselt, and local mass transfer and diffusion coefficients, as well as local
    Sherwood and Schmidt numbers, and a mysterious parameter BETAx :)

    When Eq. 14.52 is compared to the numerical results from Sparrow et al.
    [42] for Sc = 0.55, the agreement is very good... Equations 14.52 and 14.53
    may be used to solve for the heat transfer coefficient iteratively. The
    details are provided by V.P. Carey [in Liquid-Vapor Phase Change Phenomena,
    Hemisphere Publishing Corp., New York, pp 378-389, 1992.]
    Equally useless? :)

    Nick
     
  7. m Ransley

    m Ransley Guest

    As far as efficeny not being reduced because of dirty coils , this is
    exactly the cause and documented so as to a reduction in efficiency of
    all types of heat exchangers. It the simplest and most common cause of
    reduced eficency. A hydro 4 watt turbine. apples and oranges.
     
  8. daestrom

    daestrom Guest

    Do you have a link for the actual studies?
    I suspect the studies were comparing 'spotless' coils with those that are
    only moderately dirty. But they do admit that dirt affects the heat
    transfer. *IF* the dirt is built up enough to prevent normal operation,
    then it's past time to clean them. The studies suggest that cleaning them
    is not something to get all obsessed with though. I've found with a couple
    of cats and kids, twice a year is plenty adequate.
    Guess you and I have a different idea of what 'full' is. I've been
    picturing something like my upright side-by-side freezer/fridge. And
    thinking of the freezer side. When we stock up, it is pretty packed with
    few free surfaces and a very low free volume. You seem to be thinking of a
    fridge with a few soda bottles and a gallon of milk. Not the same 'full'.
    No, I think of a freezer that is 'full' as one with a months worth of frozen
    foods and meats. No significant air space between the cartons on the
    shelves, just a 1/2 gap to the bottom of the shelf above. The meat packages
    in a pull out drawer that is 10" tall and the width and depth of the freezer
    (times 2 drawers)
    You assume only 4 ft^3 of 'stuff' for a 'full' freezer? That ain't 'full',
    that's poorly packed. I'm thinking more along the lines of 25-28 ft^3 of
    'stuff' in a 30 ft^3 freezer. Now maybe you see a different point of view?
    Your data doesn't support or refute any position. It is only one test of a
    'not full' refridgerator. The mirror was obviously warmer than the fridge
    when you put it inside, so any air (with moisture) that entered the unit
    while you did this condensed on cooler surfaces. If it's a combination
    fzr/fridge, some of the air was circulated into the freezer compartment and
    some moisture was condensed there as frost.

    Leave the mirror in the unit for several hours, then open the door and leave
    it open as you have suggested. If the mirror is on the upper shelf, I would
    predict it will fog then. But all this would prove is that moisture enters
    a 'not full' unit and condenses on cold things. Not much of a revelation.
    Yes, when modeling the condesation inside a power plant's steam condenser we
    find that a small amount of 'non-condensibles' has a *major* impact on the
    overall heat transfer coefficient. A 'small' air leak of only 100 SCFM is
    within the capacity of the 'air-removal' systems operating at ~27 inhg. But
    the reduction in heat-transfer is enough to cause a loss of vacumn and trip
    the turbine at 22 inhg. If the power can be reduced quickly enough (to
    reduce the steam flow and heat load), the vacumn can be maintained at
    ~27inhg.

    "Principles of Heat Transfer" pg 524 explains that normal condensation on a
    vertical surface forms a film of water that flows laminarly down the face.
    Further condensation occurs by conduction through the laminar layer to the
    wall. By placing condensing tubes horizontally, the water can be shed from
    the tubes and thus have a smaller average film thickness. This results in a
    better overall heat transfer coefficient. Hence, most tube-shell condensers
    work best with horizontal tube designs.

    A second big effect is creating turbulance in the water film. With a higher
    vapor velocity, moving downward across the tubes, the water film layer can
    be much thinner and improve heat transfer.

    Later, in ch 12, they explain how the mass distribution of a vapor-air
    mixture will cause the mixture to flow towards the tubes. But at the tubes,
    the vapor is removed by condensation and the air 'piles up' (my words, not
    theirs). So although the partial pressure of the non-condensibles might be
    as little as 0.5% of the total pressure at the inflow point, it can be much
    higher at the water film surface (on the order of >50% of the total
    pressure). This air, carried to the tube surface by the flow of the
    condensible vapor towards the tubes, blankets the tube with a layer of air.
    This layer interferes with the heat transfer.

    One particular form of condensation with extremely high conductance is
    'drop-wise' conduction. If the surface of the tubes can be specially
    treated so as to not be 'wetted', the condensed vapor will form large
    droplets and quickly fall from the surfaces without covering them with a
    film of water. Unfortunately, this is hard to achieve outside the
    laboratory and hasn't found any wide spread use.

    For a unit length horizontal tube of diameter D, Rohsenow found the average
    film conductance ...

    hc = 0.725*( (rhol*(rhol-rhov)*g*hfg*k^3) / (D* mul*(Tsv - Ts)) )^0.25

    For vertical flat plates of height L and unit width...

    hc = 0.943*( (rhol*(rhol-rhov)*g*hfg*k3) / (L*mul*(Tsv - Ts)) )^0.25
    I don't have any direct experience with LiCl, but it may be similar to LiBr
    (certainly they would be chemically similar, lithium and a member of the
    halide group). Lithium Bromide has been used in absorption refrigeration
    cycles for many years. Those units use water as the 'refrigerant' and it is
    absorbed by the LiBr solution. The LiBr units I've worked on benefited
    greatly by having a small quantity of isopropyl alcohol added to the water.
    This made the water 'wetter'. This aids the 'boiling' of the water in the
    evaporator section.

    Air in LiBr units is more of a problem with controlling the partial pressure
    of water low enough to boil at ~38F. This outweighs any heat-transfer
    concerns. Keep the absolute pressure of the unit < 0.12 PSIA and
    air-blanketing of heat transfer surfaces is a non-issue.
    FWIW, in the texts that I have, BetaK is defined as the "temperature
    coefficient of thermal conductivity". Units of 1/F, used to calculate the
    thermal conductivity of a material when it changes appreciably over the
    temperature range in question.
    k(T) = k0*(1+BetaK*T)

    Would this 'fit' where you see it used?? Perhaps it is the "temperature
    coefficient of 'x'", whatever 'x' is??
    Mass diffusion of one gas in another in the steady-state has been shown to
    be...

    Na/ A = -Dv d(Ca)/dy
    Na => lb-moles/hr of gas A
    A => area perpendicular to the diffusion
    Dv => diffusivity in ft^2/hr
    Ca => concentration of gas A in mixture
    y => distance in the direction of diffusion

    Now, when this is applied to a mixture of condensible and non-condensible
    flowing toward a cold surface, the vapor is of course condensed. But the
    air is not and must be moved away from the surface by diffusion backward
    away from the surface. After some ideal gas work, integration and more math
    than I care to repeat here, we get...

    Na/A = (-Dv*P*(Pa2 - Pa1)) / (R*T*(y2 - y1) ln (Pb2/Pb1))
    Na/A = > mass flux of condensible A toward surface
    Dv => diffusivity of two gasses in each other
    P=> Total pressure of mixture
    R=> ideal gas constant
    T=>Temperature of mixture (absolute)
    y2 & y1 => distance from surface at 2 and 1
    Pb2 & Pb1 => partial pressure of non-condensible in mixture at 1 & 2
    Pa2 & Pa1 => partial pressure of condensible in mixture at 1 & 2

    This shows that the partial pressure of the air ('B') rises sharply as you
    approach the surface and that the mass flow of vapor is a strong function of
    the diffusivity through the air.

    For the heat transfer across a unit area used in dehumidification where the
    partial pressure of vapor is small compared to the total, the heat transfer
    through the liquid film must be equal to the heat transfer through the gas
    film plus the latent heat of condensation. Since the exact conditions at
    the gas-liquid film interface are unknown, the following equation must be
    solved iteratively through trial and error.

    hl*(Ti - Tl) = hg*(Tg - Ti) + lambda*k*(Yg - Yi)

    hl => liquid film heat transfer coefficient
    hg => gas film heat transfer coefficient
    Tl => bulk liquid layer temperature
    Tg=> bulk gas temperature
    Ti=> temperature at the gas-liquid interface
    lambda => heat of vaporization/condensation
    k=>gas-phase mass-transfer coefficient
    Yg => concentration of vapor in gas (bulk)
    Yi=> concentration of vapor in gas at the gas-liquid interface
    (subscript 'g' is in the gas, 'i' is at the gas-liquid interface, and l is
    the liquid).

    daestrom
     
  9. Guest

    I seem to recall they were cited on those web pages.
    Sounds reasonable, if you mean the fridge runs all the time or it
    doesn't get cold enough or it doesn't cool warm foods fast enough.
    I've been thinking about a refrigerator, as in the subject line above.
    Something like that. Is 120 is a few? :)
    That seems like a different situation. BTW, the bottle surfaces are much
    stiffer cool sources than the fridge walls, no? They are thin and backed
    by liquids, vs a 1/16" plastic fridge wall with insulation behind it.
    The bottles would be more effective condensers, after a few seconds.

    What would you estimate, in this case? Might be a simple duct calc.

    Another duct calc?
    Just thinking about a fridge, packed as described above.
    Maybe some beer as well. And yoghurt, and tofu, and lettuce.
    IMO, it indicates that a square foot of fridge wall would absorb less heat
    than a square foot of bottled liquid, when the door is open.
    An hour before the test...
    I didn't see or feel any condensation anywhere.
    This is a top freezer (frost free) unit. I only opened the fridge door.
    As I recall, the fan that circulates air between the compartments stops
    when the door is open.
    I'll try again, altho I think 1 hour is several mirror time constants.
    I suspect the room air was too cold and dry for condensation to occur.
    I like the idea of heating a house with a LiCl pool that absorbs water vapor
    that diffuses upstairs from a damp basement floor, in the presence of air,
    and cooling a house with a water fountain or fogger inside and a LiCl pool
    on the roof that absorbs water vapor from below, in the presence of air...
    Dunno. Maybe x is a distance. The text says

    The resulting expression relates the local Sherwood number to the local
    Reynolds number, the Schmidt number, and the parameter BETAx. A second
    equation relating these quantities results from the condition that the
    interface is impermeable to the noncondensible gas.

    -1/2
    Sh Re = BETAxSc/(1-omega), where omega = Wg /Wg
    x x oo i.

    Way over my head...
    Also way over my head. How many pounds of water vapor per hour would
    condense on a 1 ft^2 vertical surface at 36 F, if it were exposed to
    70 F still air at 50% RH?

    Tg = 70 F? hl = 60 Btu/h-F-ft^2? Tl = 36 F? lambda = 1000 Btu/lb?
    Yg = w = 0.00792 pounds of water vapor per pound of dry air?
    What are hg and k and Yi? Use Ti = (Tl+Tg)/2 to start?
    What's next?

    Nick
     
  10. daestrom

    daestrom Guest

    No, they both point to, "A review of measured tests with refrigerators
    showed that there was no or little evidence of improved
    efficiency from cleaning the coils (Litt, Megowan, and Meier 1993)."

    A google search for the authors found...
    http://hem.dis.anl.gov/eehem/93/930108.html
    The study was performed on 27 units, average age of 16 years. The
    *maintenance* performed included replacing *all* gaskets and
    cleaning the coils once over the year. When doing the maintenance, only 10
    of the units actually needed their coils cleaned, the
    others were already clean (presumably the homeowner cleaned them??). In
    five cases, the air flow was severely restricted.

    A small decrease appreaed in summer energy use after the maintenance, but
    the annual averge increased by 50 kWh (2.5%). This might
    be attributed to year-to-year variations in outside temperature. Three
    showed a clear drop in usage, two showed an increase. The
    data for three of the units is plotted. The third unit, one that had a
    'plugged' condenser showed a marked drop in energy usage.

    Interesting, but hardly overwhelmingly persuasive to not clean the coils
    once or twice a year. In fact, it demonstrates that a unit
    with a severly restricted condenser coil *does* use more energy.

    Another study...
    http://hem.dis.anl.gov/eehem/93/930109.html
    Shows that old refrigerators turned in for recycling were tested. 28 units
    were sampled and had their usage measured before/after
    cleaning the condenser coils. The tests showed a 6% reduction in energy
    usage (150kWh/year). Assuming coils are cleaned once a
    year, actual savings may be one-half of the above--75kWh per year.
    Congratulations, you've contrived a very specific construction/loading
    pattern to *maximize* your argument. We could just as easily
    assume glass shelves, boxes and rectangular containers stacked with no
    intervening space and a few other details to maximize my
    argument.

    Or use two-litre bottles, or even 1 gal., 'squarish' milk jugs. Less space
    between them, lower surface to volume ratio. We could
    go on like this forever.
    Simple duct? Maybe. Wire-mesh or glass shelf so air does/doesn't flow
    evenly down the entire cross-section of the 'duct'? Duct
    between successive shelves line up forming longer channel, or staggered so
    flow is diverted from side to side?
    That was never a question, but yes it's true. It also takes less to cool
    the same wall back down again. A 'full' refrigerator can
    maintain its average temperature low for a longer time with the door open.
    If we leave the door open long enough for this to impact
    the energy usage, then maybe we need to learn how to shut the door ;-)

    If the air flow in/out of the two different cases is significantly
    different, it can sway the outcome of a 'one-minute door open'
    test more than the average surface temperature. But as we have mentioned,
    we have vastly different opinions about how the
    'stacking' of foodstuffs can affect this air flow.
    Oops, missed that earlier. 59F @40% would only be about 85% RH at 40F. No
    condensation will form from these conditions.

    An interesting idea, but how do you 'recharge' the LiCl? IIRC, you would
    have to heat it to drive off the moisture it absorbs. But
    perhaps this 'charge'/'discharge' cycle could make an effective energy
    storage mechanism for an intermittent energy source such as
    solar thermal?
    A short 'blurb' on some of these 'dimensionless numbers' (watch the link, it
    will probably wrap badly)
    http://www.fuelcellknowledge.org/re...dimensionless_groups/dimensionless_groups.pdf
    Hmm... "See reference 2: A. P. Colburn and O.A. Hougen, 'Ind. Eng. Chem.'
    Vol 26 (1934) pp 1178-1182" ;-)

    Searching google, I found this page. A couple of pages in is the Chilton
    and Coulburn analogy for k for a tube. But their study
    was for acetone in nitrogen, so the parameters are a bit different ;-)
    http://www.p2pays.org/ref/06/05796/42199/chapter4B.htm

    Most of the work in this area is experimental correlations. There are many
    studies covering air-coolers and tubed condenser systems. I
    suspect it matters a lot on the exact nature of the plate and many variables
    like surface roughness, 'wettability' of the surface,
    mixture flow and more. Searching the web, I found numerous papers on tubed
    condenser systems like the one above, but they don't
    seem directly 'on point'. I'll look in some references I have at work on
    Monday if I get a minute.

    daestrom
     
  11. I really can't see how they figure that. It's only valid (AFAICT) if
    the tested units had not been cleaned in two years, when in fact they
    may have never been cleaned.

    It might have been more interesting to see a breakdown of how many
    units (with how dirty a condenser, if they could quantify that) saved
    how much after cleaning.

    /*
    Seventy percent of the units had a substantial dirt accumulation on
    the condenser coils, 34% had an over or under-charged refrigerant
    level, and 18% had some type of door seal or cabinet damage.
    */

    A fridge with low charge and dirty coils might not "save" a lot from
    having it's coils cleaned, but that doesn't mean cleaning the coils
    isn't a good way to save money on a fridge in otherwise good
    condition.

    In general, the blanket statement "Cleaning coils does no good,
    because we couldn't prove it with _this_ study" is probably wrong,
    given that common sense tells us that at the extreme case (condenser
    air flow stopped due to dirt) will benefit from cleaning. Obviously,
    cleaning blocked coils will help, and cleaning clean coils will do no
    good, so the real question is "How much dirt is bad, and how do we
    tell?" Fridges _could_ have a few temperature sensors that would
    allow a "Clean Coils" light, but AFAIK none do...
     
  12. Guest

    Nice research. If 50 kWh was 2.5% of the annual average, each one
    used 2000 kWh/year? Some new fridges use about 400 kWh/year...
    All's fair in love and refrigeration.
    Sure. We might also ask how most people pack fridges, or agree
    to stop arguing about that and discuss one definite situation.

    Based on Steve Baer's observation that cool air is "sluggish" when
    it naturally flows through channels less than about 1/4" wide...
    I'd say "wire mesh" and ignore it, along with details like the tapered tops
    of the bottles.
    Lined up, like 5' vertical cylinders.
    I'd say yes...
    I'm not sure how the latter matters.

    An empty massless fridge might exchange exactly one volume of air when
    the door is opened, with no further flow, since the temperatures inside
    and outside would then be equal. The penalty for opening the door would
    be the energy needed to cool that single volume of air and condense its
    vapor to the dew point.
    Which makes for a continuous temp diff and a continuous flow and
    more sensible and latent heat gain while the door is open.
    It seems to me that even a few seconds has a significant impact.
    How fast does moisture from room air condense? Full fridges packed
    as above are better condensers...
    One way is to trickle it over a dark roof. These guys used a latent
    heat exchanger with no air and no compressor to recover the coolth.
    I don't quite understand this paper. You might, and estimate how big
    an open-air pond must be to produce a given heating or cooling power.

    ***

    "Unglazed collector/regenerator performance for a solar assisted open cycle
    absorption cooling system" by M. N. A. Hawlader, K. S. Novak, and B. D. Wood
    of the Center for Energy System Research, College of Engineering and Applied
    Sciences, Arizona State University, Tempe, AZ 85287-5806 USA, in Solar Energy,
    Vol. 50, pp 59-73, 1993 describes: "An ordinary black shingled roof... used
    as a collector/regenerator for the evaporation of water to obtain a strong
    solution of [lithium chloride] absorbent... Experimental results [using a
    36' x 36' roof] show a regeneration efficiency varying between 38 and 67%.
    Cooling capacities ranged from 31 to 72 kW (8.8 to 20 tons)", ie about 1 ton
    per 100 square feet of roof area.

    In the house "water [the refrigerant] is sprayed into an evaporator, evacuated
    to about 5 mmHg of pressure, where it immediately flashes into vapor... Cold
    water, pumped from the bottom of the evaporator, flows through a fan coil...
    that blows cool air into the conditioned space. The absorber acts as a vapor
    compressor and condenser for the system. Water vapor from the evaporator flows
    over the absorber where it is absorbed by the concentrated absorbent. The
    continuous absorption of water vapor maintains a low pressure in the system
    and permits flashing of water in the evaporator... The product of the
    absorption process, a weak absorbent solution, collects at the bottom of
    the absorber to be pumped [up over the roof] for concentration."

    "The dilute LiCl solution was delivered to the collector surface through
    a spray header spanning the top of the roof and made from 50.8 mm (2 in)
    diameter CPVC pipe fitted with 35 evenly spaced brass nozzles. The
    concentrated solution collected at the bottom... in a PVC rain gutter, and
    returned via gravity feed to a 1608 l (425 gallon) fiberglass tank... In
    the event of of a rain, fluid flowing off the collector could be manually
    diverted to a 946 l (250 gallon) wash tank or to a roof drain. During the
    initial phase of the rain, residual salt would be washed from the roof
    and collected in the wash tank to be stored for later regeneration. After
    sufficient rainfall, the rainwater is diverted to the roof drain."

    ***
    Yes... 130 F seems like enough, in practical terms. The heating might
    happen in a multi-effect rooftop solar still with transparent trays
    arranged so evaporation from lower trays condenses on trays above...
    Sounds like a great idea, compared to 180 F water. No loss of heat over time,
    even with no insulation, and up to 10X less volume for the same heat storage,
    since a pound of LiCl can absorb about 10 pounds (10K vs 1K Btu) of water...
    http://www.fuelcellknowledge.org/re...dimensionless_groups/dimensionless_groups.pdf

    Thanks. I'll go look.

    I almost understand that.
    I don't think I have vol 26 of Ind. Eng. Chem.
    Maybe perfectly smooth, perfectly wet, and perfectly mixed is simpler.
    Thanks.

    Nick
     
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