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TD of Refrigeration Evaporators
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
The saturation temperature is the temperature at which the refrigerant will be at a given pressure if it is currently 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. We will also 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.
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 is 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 (5.5K) of superheat is average, with a range from 3°F to 12°F (1.65K-6.6K) depending on the equipment type (10°F (5.5K) for medium-temp, 5°F (2.75K) for low-temp, 3°F (1.65K) 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. (We talk more about that in THIS article.) When a piece of refrigeration equipment is in a hot pull down, it cannot be expected to abide by the typical DTD or superheat rules. It 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 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 the product. Keep in mind that NOTHING is a substitute for manufacturer's data, but there are some good DTD guidelines for traditional/older refrigeration equipment below. Keep in mind that the trend is toward lower evaporator TD on newer equipment.
Walk-ins 10°F DTD +/- 3°F
Reach-ins 20°F DTD +/- 5°F
A/C 35°F DTD +/- 5°F
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 (2.75K-5.5K) of its 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 sight glass.
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 +/- 3°F.
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 DT + 10°F superheat = 35°F suction line temperature +/- 3°F
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 accuracy of your box thermometer, line thermometer, and the assumed saturation temperature, you would still expect a 35°F (1.66°C) suction line temperature +/- 3°F (1.65K).
For a -10°(-23.33°C) box, low-temp reach-in, you would calculate it this way:
-10°F- 20°F DT + 5°F superheat = -25°F suction line temperature +/- 5°F
Clearly, this is NOT the way to commission a new piece of equipment or benchmark a system you haven't worked on before. Nevertheless, 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, TDs have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.
This means that TDs as low as 5°F can be designed into some units, but keep in mind that 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.