Tag: enthalpy

Have you ever noticed that the more you’re required to speed up to get all your work done in a day, the more the cleanliness of your work vehicle suffers?

Some techs won’t clean their vans no matter how slow or busy the schedule gets, but for most of us, we prefer a clean and organized vehicle but in the really busy season it can slip a bit.

This is entropy at work

One way to think of entropy is the tendency for energy to be “wasted” or for energy to become more disorganized as it is changed from one form to another. In practical terms the higher the entropy the greater the waste in moving energy around and the lower the entropy the less energy is wasted or unusable.

When your van is clean you waste less energy trying to find what you need but that is easier to do when you aren’t “rushed”.

Don’t confuse entropy with enthalpy. Entropy is the measure of the “availability” of energy to do useful work, enthalpy is a measure of total heat content.

Another way to think of entropy is simply the tendency for everything to go from more organized to less organized over time.

When we talk about entropy in classroom settings when analyzing a pressure-enthalpy diagram of a refrigeration circuit like the one shown above. You will notice the red shape is nearly a rectangle other than the slanted line on the right that shows what happens when the refrigerant is compressed. Rather than going straight up and down it curves to the right to show that additional heat is added to the refrigerant (enthalpy) as it is compressed. This increase in enthalpy follows something called lines of constant entropy, in other words, as more energy is added to a system the faster the molecules move and the less organized they become.

As pressure and temperature increase, so (generally) does entropy, just like when it get’s hot and the dispatcher starts putting the pressure on you your van entropy also increases.

In a refrigeration circuit, the system will work more efficiently when we achieve the desired movement of BTUs with a minimal amount of entropy. Practically speaking this means using lower pressures and temperatures to move heat from one place to another when possible, but we still need to get the job done of moving heat from one place to another and in that process, entropy WILL occur.

We all know the guy who has the PERFECT van, the spotless tools and the clean uniform. His van stays that way because he hardly does anything and then he takes 20 minutes after every call wiping down and oiling his tools. He has very little entropy in his van because there is very little WORK being done.

Absolute zero is the temperature at which no molecular motion exists, in that state there is also no entropy. No heat, no work, no disorganization. More heat, more work, more disorganization (entropy).

So don’t use me as an excuse for a messy van, but if your boss hassles you too much on a busy day you can remind him. Heat and pressure result entropy and the back of that van… that’s entropy baby.

— Bryan

P.S. – For those of you engineering types I know that this article took some liberty with definitions. It’s an analogy, not a doctoral physics thesis.

Just so you don’t get bored and quit reading let’s go straight to the point.

When the blower runs for more than a few minutes after the system has cycled off in cool mode the air may continue to be “cooler” (lower sensible temperature) coming out of the supply but the heat content of the air will remain unchanged. 

The only reason I say “may” be cooler instead of “will” be cooler is that we are assuming there is moisture on the coil and/or in the pan and the indoor RH is less than 100%.

Translation: When you run the blower once the system has gone off in cool mode you will continue to cool for a while, but that extended cooling comes from the evaporation of water out off of the coil and out of the pan. This results in sensible cooling and greater sensible efficiency but also increased indoor humidity.

Translation of the translation: It may feel cooler but there ain’t any less heat in the air by the time you figure for humidity.

Translation of the translation translation: If you live in a humid place run shorter off-cycle run times and think twice before running the fan in the “on” position. If you are in a dry place then let it blow until your heart is content.

Whenever cooling occurs by direct evaporation of a substance into an airstream (think a swamp cooler) it occurs at no net decrease to the heat content in the air. The heat is just going from sensible (what you can measure with a thermometer) to latent resulting in higher relative humidity air.

If you go below this line it is going to get nerdy… BEWARE


Now let’s talk about why, but first some terms.

Heat = Molecular energy or total molecular movement within a substance
Temperature = Molecular velocity, the speed that the molecules are moving
Adiabatic Process = A change in temperature without a change in heat content

Think of adiabatic process like this – You have a whole room full of ping pong balls bouncing around in a zero-gravity room. The balls are molecules, their total motion is the amount of heat and the speed they move is temperature. If you were to change the size of the room by bringing in one of the walls the balls the balls would bounce faster because the available space was decreased so the “temperature” would increase but the number of balls and the total motion would remain constant (this is what happens to refrigerant in a compressor by the way). If you were to move a wall outward and increase the size of the room the speed of the of the molecules would decrease, resulting in less speed and lower “temperature”. All the while the number of balls and the total motion remained constant (which is what occurs at the outlet of the metering device). In both of these examples temperature (Sensible heat) changes but the total heat content does not change, these are both examples of an adiabatic process due to compression and expansion of contained molecules.

An adiabatic process can also occur in uncontained systems like open airstreams, and evaporation of water is one such example.

Evaporation of water is a process where heat is absorbed into water molecules as they evaporate from liquid water and become entrained in the air as a vapor displacing some of the nitrogen and oxygen in the air. When that heat is absorbed from the air into the water it results in lower sensible temperature, but the water is still CONTAINED IN THE AIR. This means that while the air may be cooler it still has all the heat contained in it in the form of water vapor.

Now for the real shock..

Water vapor is NOT more dense than dry air at the same temperature it is actually less dense / lighter than dry air, however, is does contain more heat (enthalpy for you nerds like me). This means that when you run the blower after a cooling cycle the moisture on the coil and in the pan are evaporated back into the space and depending on the RH of the air it will lead to sensible cooling but latent gains. This means cooler but higher RH and this is due to the higher heat content of higher RH air at the same temperature.

Once again, depending on where you live this may be positive or negative.

In Arizona or Colorado? Run that blower after the cooling cycle.

Florida? May wanna shut it off right after the cycle or maybe 90 seconds at most and leaving the fan in the “on” position will likely result in a small increase of indoor RH.

— Bryan

 

 

P.S. – I also did a Facebook Live Video about it today

also… Here are some great videos on the subject by Jim Bergmann

This article is written by my buddy and Canadian Supertech Tim Tanguay. Thanks Tim!


This P/E chart shows R410a at 100°F Saturated Condensing Temp, 10°F SC
40°F Saturated Suction Temp, 20°F SH at the compressor.
The green highlighted thumb shape is the saturation zone. Everything that occurs in the saturation zone is a latent (change of state) process.
Everything that occurs to the right (superheating) and left (subcooling) is a sensible process.
Go to the movies in your mind, imagine that you are one pound of 410A. We commence our journey at the rightmost point on the upper orange highlighted line.
At this point, you have just left the compressor. You are a superheated vapor, with a temperature of 145°F. You enter the condenser and start rejecting heat to the atmosphere. After rejecting 45°F of sensible heat (desuperheating), you hit the saturated condensing zone (100°F) and you turn into a drop of liquid. As you march your way along the condenser (follow the line left), you reject latent energy but stay the same temperature. As your latent energy decreases you become more liquid until finally, you are a solid column of liquid and you exit the saturation zone to the left of the thumb. You then give up another 10°F of sensible heat to the air and become a 90°F subcooled liquid.
You approach the sight glass as a 90°F subcooled liquid under approximately 350 PSIG of pressure. As you pass the sight glass, you fart a few bubbles just to mess with the refrigeration mechanic observing the process. You squeeze your way through the tiny orifice in the metering device and emerge into the evaporator, solidly back into the saturation zone. You find yourself as a 40°F saturated liquid at 125 PSIG (approx 78% liquid, 22% vapor, indicated along the constant quality lines).
 Now you make your way along the bottom line towards the right side of the thumb, you absorb heat energy from the warm return air rushing over the copper and aluminum evaporator fins. The heat you absorb boils you dry. You are naught but a vapor, and as such, the energy from the return air increases your sensible heat by 10°F. You emerge from the evaporator as a 50°F superheated vapor. As your journey progresses
towards the suction inlet of the compressor, you pick up another 10°F of sensible heat.
You enter the suction port of the compressor as a 60°F, superheated vapor. The compressor puts you through a strenuous workout, squeezes you into a smaller volume and in the process increases your temperature by about 85°F.
You emerge as a superheated 145°F vapor. The process begins anew.
A few things to look at. The numbers on the top represent enthalpy energy, as BTU’s per pound.  In this particular example, the sensible portions of the condenser account for approx 20% (eyeball estimate) of the total heat rejected in the condenser. The other 80% of the process is latent.
On the right-hand side of the PE diagram, you have specific volume, represented as curved dotted lines. As SST decreases, specific volume increases and vapor density decreases. This fact alone is why refrigeration compressors need to be physically larger. As specific volume increases, the volumetric efficiency of compressors decrease.   Lower SST’s (suction saturation temp) require larger compressor displacement because they need to move more gas to obtain the required mass flow. In AC and refrigeration, the mass flow of refrigerant through the system ultimately determines your system capacity.
At 40F, the latent heat of vaporization of 410A is approx 75 BTU/LB. Compare that to water, which has a latent heat of vaporization of approx 970 BTU’S per pound at 212°F/14.69 PSIA and you begin to realize why dehydration of a system takes so darn long.  It takes a LOT of energy to boil water off.
In the evaporator, about 10% of the process is sensible.  This is why a unit that is short on refrigerant isn’t able to cool properly. The refrigerant boils off leaving a large portion of the coil to collect sensible heat (higher superheat). The amount of heat that sensible processes remove from the air stream is relatively tiny, thus we lose capacity.
So too with things like water. The sensible heats involved with changing temperature are minuscule when compared to the amount of heat required to change state. Universally, latent changes require orders of magnitude more energy than sensible changes.
— Tim

Both wet bulb temperature and air enthalpy are extremely useful to understand when calculating actual system capacity as well as human comfort. Dry bulb temperature is a reading of the average molecular velocity of dry air, but it does not take into account the actual heat content of the air, or the evaporative cooling effect of the air.

Like we mentioned in the last tip, when air is at 100% relative humidity the dry bulb, wet bulb and dew point temperatures are all the same. This is because at 100% relative humidity the air is completely saturated with moisture and can have no evaporative effect.

When air is less than 100% RH it will provide an evaporative cool effect and wet bulb temperature is a measurement of that effect. In fact, wet bulb temperature is the temperature a damp thermometer bulb will read when exposed to a 900 FPM (Feet per minute) air stream. If you have ever seen someone using a sling psychrometer, that is exactly what is happening (Hopefully you have a wrist that is well calibrated to 900 FPM). The lower the wet bulb in comparison with the dry bulb (This differential is called wet bulb depression) the lower the relative humidity and the greater the evaporative cooling effect.

Enthalpy is the total heat content of the air and is represented in BTUs per lb of air. By converting lbs of air to cfm we can calculate the amount of heat in an air mass as well as the change in the enthalpy across a coil to calculate the heat moving capacity of a coil, BTU losses/gains over a length of duct and much more.

You will notice that wet bulb and enthalpy are slanted lines, descending from left to right and they are equivalent. This means that a particular wet bulb temperature is also equal to a particular enthalpy (At 14.7 PSIA at least). In the chart above you can see that a 62.8 degree wet bulb mass of air contains approximately 28.4 BTUs per lb. The tricky part is reading at this extreme level of resolution, because 28.4 vs. 28.6 can make a significant difference when it is multiplied out over a large air mass. This demonstrates why VERY accurate tools and careful calculations are required for enthalpy calculations in HVAC/R.

— Bryan

For a full WB Enthalpy calculator go HERE and look for the enthalpy chart

Enthalpy is easy… it’s just a state function that depends only on the prevailing equilibrium state identified by the system’s internal energy, pressure, and volume. It is an extensive quantity. Simple.

Like most things, the scientific definition is as clear as mud. In HVAC/R we use enthalpy measurement to come up with the total heat change in a fluid, whether it’s refrigerant, water or air.

That total change in heat content or enthalpy change is called Delta H (ΔH) which is just another way of saying “total heat split” and it is generally measured in BTU/lb in the US.

In air, we need to use probes that measure humidity and temperature like the HUB2 probes shown above or the Testo 605i probes in order to calculate the enthalpy of the air. Air has both the energy associated with the temperature of the air as well as the latent heat stored in the water vapor.

UEI HUB Screenshot

If you want to use the Δto calculate the total heat added or removed from the air you would then use this formula to calculate BTUs of heat added or removed from the air.

Total Heat = (H1-H2) x 4.5 x CFM

In the case above it would be

Total Heat = (29.68 – 22.77) x 4.5 x 730 (CFM we measured)

so

29.68 – 22.77 = 6.91 Δ

6.91 x 4.5 x 730 = 22,699.35 BTU/hr

This total air enthalpy change is a required part of calculating total system capacity and is a pretty simple thing to understand.

Don’t confuse ΔH (Total Heat Change) with ΔT (Temperature Difference). ΔH includes both latent and sensible heat and is a measure of heat quantity in BTU/lb while ΔT only calculates temperature difference and isn’t converted to BTUs at all.

— Bryan

In HVAC/R we are in the business of moving BTUs of heat and we move these BTUs on the back of pounds of refrigerant. The more pounds we move the more BTUs we move.

In a single stage HVAC/R compressor, the compression chamber maintains the same volume no matter the compression ratio. What changes is the # of pounds of refrigerant being moved with every stroke(reciprocating), oscillation (scroll), or rotation (screw, rotary) of the compressor. If the compressor is functioning properly the higher the compression ratio the fewer pounds of refrigerant is being moved and the lower the compression ratio the more pounds are moved.

In A/C and refrigeration the compression ratio is simply the absolute discharge pressure leaving the compressor divided by the absolute suction pressure entering the compressor.

Absolute pressure is just gauge pressure + atmospheric pressure. In general, we would just add the atmospheric pressure at sea level (14.7 psi) to both the suction and discharge pressure and then divide the discharge pressure by the suction. For example, a common compression ratio on an R22 system might look like-

240 PSIG Discharge + 14.7 PSIA = 254.7
75 PSIG Suction + 14.7 = 89.7 PSIA
254.7 PSIA Discharge ÷ 89.7 PSIA Suction = 2.84:1 Compression Ratio

The compression ratio will change as the evaporator load and the condensing temperature change but in general, under near design conditions, you will see the following compression ratios on properly functioning equipment depending on the efficiency and conditions of the exact system.

In air conditioning applications compression ratios of 2.3:1 to 3.5:1 are common with ratios below 3:1 and above 2:1 as the standard for modern high-efficiency Air conditioning equipment.

In a 404a medium temp refrigeration (cooler) 3.0:1 – 5.5:1  is a common ratio range

In a typical 404a 0°F to -10°F freezer application 6.0:1 – 13.0:1 is a common ratio range

As equipment gets more and more efficient, manufacturers are designing systems to have lower and lower compression ratios by using larger coils and smaller compressors.

Why does the compression ratio number matter? 

When the compressor itself is functioning properly the lower the compression ratio the more efficient and cool the compressor will operate, so the goal of the manufacturer’s engineer, system designer, service technician and installer should be to maintain the lowest possible compression ratio while still moving the necessary pounds of refrigerant to accomplish the delivered BTU capacity required.

The compression ratio can also be used as a diagnostic tool to analyze whether or not the compressor is providing the proper compression. Very low compression ratios coupled with low amperage and low capacity are often an indication of mechanical compressor issues.

Compression ratio higher than designed = Compressor overheating, oil breakdown, high power consumption, low capacity 

Compression ratio lower than designed = Can be an indication of mechanical failure and poor compression

Understanding compression is critical to understanding the refrigeration process. Don’t be tempted to skip past this because it is a really important concept.

Look at the pressure enthalpy diagram above. Top to bottom (vertical) is the refrigerant pressure scale, high pressure is higher on the chart. Horizontal (left to right) is the heat content scale, the further right the more heat contained in the refrigerant (heat, not necessarily temperature).

Start at point #2 on the chart at the bottom right. This is where the suction gas enters the compressor. As it is compressed it goes to point #3 which is up because it is being compressed (increased in pressure) and toward the right because of the heat of compression (heat energy added in the compression process itself) as well as the heat added when the refrigerant cooled the compressor motor windings.

Once the refrigerant enters the discharge line at point #3 it travels into the condenser and is desuperheated (sensible heat removed). This discharge superheat is equal to the suction superheat + the heat of compression + the heat removed from the motor windings. Once all of the discharge superheat (sensible heat) is removed in the first part of the condenser coil it hits point #4 and begins to condense.

Point #4 is a critical part of the compression ratio equation because the compressor is forced to produce a pressure high enough that the condensing temperature will be above the temperature of the air the condenser is rejecting its heat to. In other words, in a typical straight cool, air cooled air conditioning system the condensing temperature must be higher than the outdoor temperature for the heat to move out of the refrigerant and into the air going over the condenser.

If the outdoor air temperature is high or if the condenser coils are dirty, blades are improperly set or the condenser coils are undersized point #2 (condensing temperature) will be higher on the chart and therefore will put more heat strain on the compressor and will result in lower compressor efficiency and capacity.

As the refrigerant is changed from a liquid vapor mix to fully liquid in the condenser it travels from right back left between points #4 and #5 as heat is removed from the refrigerant into the outside air (on an air cooled system). Once it gets to #5 is is fully liquid and at point #6 it is subcooled below saturation but ABOVE outdoor ambient air temperature. The metering device then creates a pressure drop that is displayed between points #6 and #7. The further the drop, the colder the evaporator coil will be. The design coil temperature is dictated by the requirements of the space being cooled as well as the load on the coil but the LOWER the pressure and temperature of the evaporator the less dense the vapor will be at point #2 when it re-enters the compressor and the higher the compression ratio will need to be to pump it back up to point #3 and #4,

This shows us that the greater the vertical distance between points #2 and #4 the higher the compression ratio, which means that both low suction pressure and/or high head pressure result in higher compression ratios, poor compressor cooling, lower efficiency and lower capacity.

In some cases, there isn’t much that can be done about high compression ratios. When a customer sets their A/C down to 69°F(20.55°C) on a 100°(37.77°C) day they will simply have high compression ratios. When a low temp freezer is functioning on on a very hot day it will run high compression ratios.

But in many cases, you can reduce compression ratios by –

  • Keeping set temperatures at or above design temperatures for the equipment. Don’t be tempted to set that -10°F freezer to -20°F or use that cooler as a freezer
  • Keep condenser coils clean and unrestricted
  • Maintain proper evaporator airflow
  • Install condensers in shaded and well-ventilated areas

Keep an eye on your compression ratios and you may be able to save a compressor from an untimely death.

— Bryan

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