Tag: saturation

We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again

Suction Saturation Temperature

Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.

Evaporator TD and DTD

Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.

Delta T

Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.

Target Box Temperature 

The temperature the refrigeration box should maintain when the system is operating properly

Superheat

The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat  is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).

Hot Pull Down

Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.

On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.

Design Temperature Difference (DTD)

In air conditioning applications a 35°F(1.66°C) DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.

There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.

Walk-ins  10°F(5.5°K) DTD +/- 3°F(1.65°K)
Reach-ins  20°F(11°K) DTD +/- 5°F(2.75°K)
A/C 35°F(13.75°K)) DTD +/- 5°F(2.75°K)

You then subtract the DTD from your box temperature / return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.

For Example –

If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of  25°F(13.75°K) +/- 3°F(1.65°K)

When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –

35°F – 10°F(19.25°K – 5.5°K) DT + 10°F(5.5°K) superheat = 35°F(19.25°K) suction line temperature +/- 3°F(1.65°K)

In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)

For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way

-10°F- 20°F(-5.5°K to -11°K) DT + 5°F(1.65°K) superheat = -25°F(-13.75°K) suction line temperature +/- 5°F(1.65°K)

Clearly, this is NOT the way to commision a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.

The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.

It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.

This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.

— Bryan

 

 

 

We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the azeotropic refrigerant mixture.
Well that wasn’t very helpful, was it? All we did was introduced new terms without defining them and further confused the issue.
So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor–liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, as far as we’re concerned, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures.
That temperature range is called the glide.

 

Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears.
Still making sense? I hope so.

 

Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.

 

What does this mean for the service guy?

 

Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.
In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.

 

Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right?
Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose?
The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.

 

52.5+40 = 92.5. 92.5/2=46.25

 

Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

–Jeremy Smith

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