What type of temperature is in evaporator?

A fluid, known as a refrigerant, moves between four key stages in refrigeration cycle. As it does so, it changes in pressure and temperature, this allows the fluid to absorb heat from one place and discharge it in another.

For a refrigeration cycle to work, it requires five main components. The four key stages of temperature and pressure change occur within these components.

• The compressor
• The condenser
• The expansion valve
• The evaporator
• The pipework which connects them all

To cool a room down, you need to collect the heat and dump it somewhere else. The air in this “somewhere else” must be a lower temperature than the refrigerant for you to be able to dump the heat. To make sure this is possible, the refrigerant is compressed so that the temperature increases. That way when the refrigerant reaches the condenser, the refrigerant within the pipe will be hotter than the air on the outside of the pipe, so it will be able to dump the heat. If the pipe was the same temperature as the air then you wouldn’t be able to reject any heat and you wouldn’t cool the room down.

The refrigerant enters the compressor as a warm, saturated low pressure gas, it is then compressed within the compressor (hence the name). During compression, the quantity of fluid remains the same but the volume decreases, this increases the pressure and temperature. The refrigerant leaves the compressor as a superheated (hot) high pressure gas.

Next this superheated high pressure gas enters the condenser, the condenser is a coil of pipework which runs between metal fins.

The metal fins help carry heat away from the pipe via conduction. A fan also blows air across the coil and fins to remove the heat via convection.

As the air is blown across the pipework and fins, it picks up the heat from the refrigerant and moves it away so that the refrigerant cools down. As it cools down the high pressure gas condenses into liquid, which is still at a high pressure.

The refrigerant enters the condenser as a superheated (hot) high pressure gas, it dumps its heat into the air being blown across by the fan, this drop in temperature condenses the refrigerant. The refrigerant leaves the condenser as a regular temperature, saturated high pressure liquid.

To cool a room down, the heat within that room must be collected and dumped somewhere else. The refrigeration cycle collects this heat by sending the refrigerant at a low temperature and pressure into the evaporator within that room.

To cool the refrigerant down it is passed through the expansion valve, this will reduce the pressure of the refrigerant by restricting how much can flow through the valve. This restriction means that there will be less refrigerant in the next section of pipe, so the refrigerant which is allowed to pass through can expand a little. This expansion reduces the temperature and gives it some storage room to collect the heat.

The expansion valve restricts the flow of refrigerant through use of an internal spring loaded valve which is connected to a diaphragm.

A thin tube, known as the capillary tube, runs between the expansion valve and a thermal bulb. The thermal bulb is in contact with the pipe just after the evaporator, the liquid/vapour inside the thermal bulb expands and contracts with the change in temperature of the refrigerant leaving the evaporator. This expansion and contraction causes the diaphragm to move which in turn controls the spring loaded valve that limits the flow of refrigerant into the evaporator.

The refrigerant enters the expansion valve as a regular temperature, saturated high pressure liquid. The expansion valve will limit how much refrigerant can pass through at one time, this results in the refrigerant dropping in pressure and temperature. The refrigerant leaves the expansion valve as a cold, saturated low pressure liquid.

The final key stage of the refrigeration cycle is the component called the evaporator. This is similar to the condenser in construction, but the refrigerant behaves differently inside.

In the evaporator, the refrigerant enters as a cold, low pressure liquid but quickly begins to boil. The refrigerant has a very low boiling temperature, typically of -23ºc (minus twenty-three degrees Celsius). As the refrigerant boils it evaporates, this evaporation picks up the rooms heat and carries it away towards the compressor where the refrigeration cycle will begin again.

The refrigerant enters the evaporator as a cold, low pressure liquid, the refrigerant begins to boil and evaporate, this evaporation causes a cooling effect in the room and the heat is carried away to be dumped in the condenser after the compressor. The refrigerant leaves the evaporator as a warm, saturated low pressure gas.

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.

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

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.

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 +/- 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.

—Bryan