Infiltration at entry

The application of "air curtains" to control infiltration at entries is common. In consideration of the infiltration/air exchange of a dock door in a particular application, I noticed that I could not physically notice any colder air "falling" out of the (colder) refrigerated space. colder space is 35degF entering 40 to 45degf space.

I estimate the air throw off of the unit coolers (blowing toward the doors from the other side of the room's ceiling) at about 80ft. The room is 35ft (across) and 25 ft high (toal linear distance across ceiling to floor = 60ft). The air movement (downward from ceiling) is noticable on the "colder" side of the entry.

In determing the infiltration rate (for the opening) for still air, 40degF @ 70%RH air compared to 28degF air @90%RH, what adjustment could possibly be made in consideration of the downward air velocity on the cold side of the opening?

In the present application, the "containment" between the two spaces seems well defined (when you are standing there). Will making the "cold" room colder (in effect making the air heavier), possibly decrease the current effectiveness of the air movement at the opening, or would this relationship NOT be proportional compared to the current application?

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In determing the infiltration rate (for the opening) for still air,
40degF @ 70%RH air compared to 28degF air @90%RH, what
adjustment could possibly be made in consideration of the
downward air velocity on the cold side of the opening?

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Not at all sure I understand your question, Herefishy. My view on infiltration is that it is pressure difference. The rate of infiltration increases as pressure differences become larger.

The question I ask myself is how to measure these pressure differences. Opening a space to different temperatures creates a pressure difference, as well as having different vapor pressures does. I don't understand how an air curtain can do a darn thing unless it discharges air into a return that treats the air. Just blowing air down over a door opening, methinks does absolutely nothing.

You have a good question there. I think that a cooler room will necessarily draw in air from the warmer room. I think that a dry room will draw in vapor from a moister room. I think they should be considered two separate processes. I go back to observations of a freezer opening up to a dock area. The way the fog rolls out from the bottom and the way the strip curtains appear to frost from the top.

Dan

Dan,

The concept to which I refer is best explained by the website www.miniveil.com and more specifically www.miniveil.com/geninf.html Even better, for a model of air curtain / air velocity application at entry, you can go direct to www.miniveil.com/select.html

You can thank GOD that I'm not going to explain it in any post!!! I have been introduced to this concept by the person who not only wrote the chapter on infiltration in the ASHRAE manual (George Smith), but who is CEO of the company HCR which manufactures vestibules (sp?) for such applications.

It being late, Herefishy, I haven't checked your sites. But I have to wonder if anything I said was incorrect; I think not. Perhaps tomorrow I will check stuff. Not. Perhaps next weekend.

Infiltration has always been the wild card in any refrigeration or air conditioning calculation. Product load and such are things you can expect the customer to explain and commit to. Not infiltration, though.

What happens when you open the door? Or leave it open?

Vestibules make such perfect sense. Perhaps that is why churches invented them.

I look forward to checking out your recommended sites. Next weekend. Fun stuff, though, Herefishy.

Well,
I'm not pushing any websites and they're not mine. I found them in a search to explain the concept relating to my question.

In regard to fun, I am pondering this scenario in an application that I am actualy working on.

I have air velocity shooting down from the ceiling resulting from the discharge of the unit coolers.

(1) How effective is this arrangement in containment of the refrigerated air at an open entry?

(2) Will the (presumed) effectiveness of this air movement be maintained proportionately as design room conditions (air enthalpy) change?

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I'm not pushing any websites and they're not mine. I found them
in a search to explain the concept relating to my question.

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I never felt you were pushing a website, sorry if I came off wrong.

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I have air velocity shooting down from the ceiling resulting from
the discharge of the unit coolers.

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I would like to know what you think the "air curtain" is accomplishing, because I do not understand.

I see no positive purpose to an air curtain unless we can talk about treating the air with it. I mean having an evaporator or some heat exchanger involved.

An air curtain, if anything, increases infiltration. Just putting a blower between different temperature or humidity zones makes small sense to me.

I apologize if I appear to be jousting with you. If you have conditioned infiltrated air, you have a good system and it makes perfect sense. If you hang a blower over a door that just blows air, maybe you keep flies out, but I think most likely you grind up the flies in the return of the blower... therefore conditioning the flies.

Dan

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Originally posted by Dan
I would like to know what you think the "air curtain" is accomplishing, because I do not understand.

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An air curtain produces counterflfow forces opposite to the two-way air exchange at doorways resulting from temperature differences.

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[/i]An air curtain, if anything, increases infiltration. Just putting a blower between different temperature or humidity zones makes small sense to me.

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The airstreams are recirculatory and travel curvilinear paths (across opening).


You're really making me work at it, Dan!

LOL. I checked out the website and it pounded some sense into my stubborn brain. The part that made most sense to me is how the air curtain entrains both the conditioned and unconditioned and splits the flow at floor level, returning mostly conditioned air back into the conditioned space and mostly unconditioned air back to the unconditioned space

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When opened a door separating a cold storage
area from a warm room permits a substantial loss
of refrigerated air. Warm air flows into the cold
room through the lower part.

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That statement puzzled me. I always thought the warm air entered through the upper part and displaced cold air left through the lower part. Hmmm.

The website indicates that an air curtain can be as much as 85% efficient. Should I take that to mean that if the expected infiltration load was 100,000 btu/hr, that the use of an air curtain could reduce that to 15,000 btu/hr? I don't think I would dare take such a large credit.

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In the present application, the "containment" between the two
spaces seems well defined (when you are standing there). Will
making the "cold" room colder (in effect making the air heavier),
possibly decrease the current effectiveness of the air movement
at the opening, or would this relationship NOT be proportional
compared to the current application?

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I found a graph which would indicate normal infiltration under your initial conditions would be somewhere around .15 tons per square foot. And increasing to about .2 tons per square foot of opening if you dropped the inside temperature to 20 deg F. (Stoecker, "Industrial Refrigeration Handbook").

But if I understand your original question, you were referring to the air curtain effect of the treated air impinging on a wall and dropping over the dock doors, and were asking if the velocity would increase if you made the air colder?

I am not sure that this is the sort of air curtain that the website is referring to. Low velocity air curtains are easily "broken" and made inefficient; if I understand your air curtain I don't think it is a "proper" air curtain as described on the website. Yours would be a gravity flow of treated air at the dock doors. Right? Or do you have air curtains on the dock doors?

Either way, I would hazard that your air infiltration load would increase by 25% if you dropped the conditioned space temperature to 20 deg F.

I might have you really working, but think how tired my stubborn brain is getting.

Between not understanding questions, overcoming my own prejudices, and reading up on stuff, I feel I have learned a thingertwo. LOL.

Thanx, dan.

Now I don't feel like an idiot!LOL!

Dan,

If I have peaked your interest, or at least broken through the ice for you on the concept, I think that you will truly enjoy reviewing this ASHRAE presentation at www.hcr-inc.com/PRES/ASHRAE/ASHRAE.htm

I think that I have concluded that the increase in infiltration will be proportional, as I believe is already demonstrated by Marc's formula, and independent load calcs by myself and Andy, and Furthermore DAN, your resources reinforce it also.

Thanx and regards,

Mark

I still haven't responded to your send about the air curtains being recirculatory and following a curvilinear path. That is a another kettle of fish, according to the website you posted. But lets now think in another dimension:

What good is an air curtain that is only in use when a forklift drives through it?

What is the efficiency of an air curtain compared to a closed door or a good dock seal?

Why do most of the air curtains I see that were installed 50% disabled? And why don't I see many air curtains in commercial warehouse applications - working or not.

Most commercial warehouses have doors that open and close within 20 seconds and most of that time equipment and loads are disrupting them.

Is it possible that air curtains are only for the foolish who choose to leave their doors open for no good reason?

Hussmann used to make a multi-deck meat case that used an ambient air curtain. Back during the design, Hussmann paid a handsome sum for a fortran program that fine tuned the design. A few years ago, the air curtain was removed from design... air velocities were lowered, and the case consumes 25% less refrigeration with better product temperature at higher air temperatures.

Go figure.

Dan

If you refer to the ASHRAE url that I posted, the old art interpreted the full velocity of infiltration being acheived at around at around 30 seconds of the opening having occured. But ASHRAE (at least) has revised through furhter experimentation that full velocity of infiltration is acheived within 3 seconds (in consideration of Newton's 2nd law - momemtum).

If you go to the url (previous) you will see the explanation of the question that you ponder.

According to the old standards, your statement is correct. But the standards have been revised, and therefore the loss/efficiency has been adjusted/changed.

Not to say at all, Dan, that you are not correct. For it is all "science" and subject to interpretation.

Mark

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Originally posted by Dan
I still haven't responded to your send about the air curtains being recirculatory and following a curvilinear path. That is a another kettle of fish, according to the website you posted. But lets now think in another dimension:

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The "miniveil" url is a salespitch. Refer to www.hcr-inc.com for more realistic representation.

Several of your initial views of the infiltration thingy regarding warm air verses cold air are correct. The miniveil site misrepresented that characteristic. You were ADEPT at identifying it!

I suggest that you you scroll back through some posts that i have made, because I have been haphazard and diligent in response to you previous.

later

...The fish.

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When asked what had changed, the response was, "the coil had been removed from its -27°F suction line to a -45°F line in
an energy-management upgrade."

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That was the first of too many "huh?'s" on the ASHRAE presentation. I have learned this much from ASHRAE: Whenever somebody says "the following should be obvious, thus requiring no explanation", I know I will not have a clue from there on. And I did not.

The paper did have some nice photos, however. LOL.

Regarding the 30 second vrs 3 second momentum thing: In my dumbfounded brain, I always thought that maximum air exchange was achieved immediately, and then tapers off. So I neither understand why the 30-second concept ever came into being, nor why you would think that things I said made sense with that mindset.

Inertia and momentum stuff, I suppose. But if you want a blast off, open a -5 deg F freezer to a 45 deg F dock area. To me, the air exchange actually should taper off within 30 seconds because all the invasive and victimized forces have formed their own vestibule of sorts.

In sum, I don't understand the question, but am not shy about answering it.:)