We don’t use refrigerant?

 

A refrigerant is a compound used in a heat cycle that undergoes a phase change from a gas to a liquid and back. The two main uses of refrigerants are refrigerators/freezers and air conditioners (cf. coolant). Since it was discovered in the 1980s that the most widely used refrigerants were major causes of ozone depletion, a worldwide phaseout of ozone-depleting refrigerants has been undertaken. These are being replaced with "ozone-friendly" refrigerants.


you should try it!

st65

 

At first, I misunderstood the question. Of course, a warm evaporator uses more refrigerant. It is needed to cool the evaporator.

Then I thought about it a little while I was surfing this site and came back to revisit this.

It comes down to gas laws. Although refrigerant vapor is not a perfect gas, it behaves closely enough for the gas laws to be applicable.

Pressure increases, temperature increases, volume of the evaporator is (relatively) constant. The density of the refrigerant vapor in the coil must increase to satisfy the gas law equation.

This is why less horsepower is required to accomplish the same amount of cooling at higher pressures and temperatures. (1hp is about 1 ton AC but only about 1/3 ton freezer)

 

nice answer jp!

 

does a cold evaporator use more refrigerant than a warm evaporator?????
is the evap of a cooler in a 38 degree box using less refrigerant because the refrigerant is cooler and more dense.

if the same box is 70 degrees? will the evap use less refrigerant?

is the refrigerant less dense at 70 and more volumetric taking up more space?

As I read the question, you're asking if the evaporator contains less refrigerant during low loads and more at high loads.

Actually, I think it's the other way around.

For a given system (in this case a 38ºF walk-in cooler system) at a high load like with the 70ºF box temp in the question, the flow rate of refrigerant (lbs/min) is greater but the velocity through the evaporator and the volume of flash gas generated are also greater. At such high pulldown loads, the TEV often is undersized at that point and youy'll see high superheat as well, meaning there's alot of flash gas in the evap. So at any given time at a high load, the toatal amount of refrigerant contained in the evaporator is less than if the load were lower.

Think of it this way. As the load decreases (ie; the box temperature drops), everything slows down. The flow rate is reduced, the volume of flash gas and the velocity drops but the TEV is now holding the proper 8-10ºF superheat at the outlet. So as the load continues to fall, the evaporator tends to load up with more liquid and less flash gas.

Carrying this scenario even further, you can then see that if the load were reduced even more (like if the box were outdoors on a cold night) the TEV may then actually be oversized for the conditions and unable to maintain SH, floodback to the compressor may occur.

So if I read the question right, I'd say the evaporator has more refrigerant in it when cold.

 

I agree with you. It comes down to mass flow rate and also the type of metering device. If it's a cap tube the pulldown will take longer. However at steady state the cap tube system may have more liquid refrigerant in the evaporator depending on the cap size and refrigerant charge.

 

I took it as being 2 "identical" evaps operating under 2 different conditions.

 

Essentially that's how I'm looking at it.

The same box, same system, same total amount of refrigerant....just looking at two different sets of conditions...one at high pulldown load and the other at normal, or low loading.

 

Ok. I wasn't clear.

Box #1. 38 degree box.(Cold evap)
Box #2. 70 degree box.(Warm evap)

Evaporator pressures correspondingly increased, therefore refrigerant density increases and there would be more refrigerant in the coil under all operating conditions.

Maybe I took the question in a different direction than was intended.

Nothing like an interesting question. Thanks to supertek.

 

I took the question a different way, being a supermarket guy under full load (lets say after an off time defrost) we have more gas or more density at a constant evap pressure (epr) an excess hp. The txv opening at the begining and closeing down as we reach box temp trying at all times to maintain superheat. The gpm has increased to match the load. The eepr does this even better giveing a lower suction pressure at high load, the txv sees this as well as the high suction line temps and opens further recovering even faster from high load.

 

I think that the point that Ice is getting at is that, at high loads, there will be less liquid, leading to less total refrigerant in the evaporator under those conditions. Lower loads there will be more liquid leading to a larger total volume of refrigerant.

 

After reading an d re-reading your initial reply to this, I realized that I was wrong and understood the answer.

As I said, this site is great for discussions like this. It really helps to understand the systems we work on better. Helps you to "get in the pipes"

 

No technical discussion like this is truly final or complete without a word (or two) from Andy Schoen from Sporlan.

So now that I've invoked his name, he should be around any time now.

(It's kinda like sending out a call for Batman.)

 

LOL.I'm sure Andy will be along soon.

 

Excellent topic. I learned something new.



I guess the Joker must have killed Batman, he never showed up.

 

LOL!!!!!!!!!!!:D:

Excellent topic. I learned something new.



I guess the Joker must have killed Batman, he never showed up.

 

the answers are clear as day when you look at a P-E chart.

low pressure=less dense ........ increase in specific volume

hi pressure =more dense........decrease in specific volume.

Refrigerant does NOT act like an ideal gas in saturated conditions (or else we be outa jobs).

 

I dont have much exp with freezers and coolers. Are the Sub cooling and Superheat targets the same as air conditioners and heat pumps?

 

Crackertech,

I have an AD in HVAC, AD in Industrial Electronics, Residential electrical license, and journeyman electrical card. Will I quailify to be a pro member after 15 post?

 

That would be up to the Membership committee.But I don't see why not.

 

You know its slow when 10 month old posts are dragged up from the depths.

 

Have ya looked around some are 4+ years.:eek2:

 

good thread, simple question quickly combining 3 big topics pulll down , density , and mass flow all 3 correllate, constantly fluctuating conditions, there will be more dense cool refrigerant in satisfied evap/system, txv closing down on less demand, thus compressors pumping less,= less mass flow, already answered here just thought to word it differently , happy new year everyone stan

 

So now it's my fault........LOL

:couchhide:

 

Ok then :beat::beat::beat: