Understanding Refrigeration Systems Humidity Control

How Evaporator Temperature Difference Affects RH

One key factor affecting relative humidity (RH) is the evaporator temperature difference (TD). This refers to the difference between the refrigerant’s saturation temperature inside the evaporator coil and the temperature of the air entering the coil, also known as the return air temperature. In this context, TD is distinct from Delta-T and Superheat—common concepts in air conditioning refrigeration systems—making it important for HVAC technicians to understand its specific role.

The Role of Evaporator Temperature Difference (TD)

The TD directly influences how much moisture is removed from the air inside the refrigerated case:

• Greater TD: More moisture is removed, resulting in a lower RH inside the case.
• Lesser TD: Less moisture is removed, leading to a higher RH in the case.

For example, a typical walk-in cooler is designed with an evaporator TD of around 10˚F (with entering air at 35˚F and a coil temperature of 25˚F). This results in an RH of approximately 85%, which is ideal for long-term food storage.

Common RH for Reach-In vs Walk-In Coolers

While cold storage rooms are designed to cool and preserve food, it’s important to distinguish between walk-in coolers and smaller refrigeration units such as display cases, merchandisers, beverage coolers, and reach-in coolers. A 10˚F evaporator temperature difference (TD) is effective for walk-in coolers, but not all refrigeration systems follow this design. Smaller units, like reach-in coolers, often have more compact evaporators, leading to a higher TD—typically around 20˚F.

Reach-in Coolers vs. Walk-in Coolers

Reach-in coolers may use an evaporator designed with a 20˚F TD such as the case for Beverage Cases or Delfield Prep Tables among others, which leads to a lower RH of about 65%. To prevent dehydration, food in these cases is either wrapped or stored for short periods—typically less than 24 hours.

These differences in TD also explain why suction pressures may vary between similar refrigeration systems. Lets make the assumption that the target box temperature is 35F and thus the return temperature is 35F. For example:

• Walk-in Cooler: If both a walk-in cooler and a reach-in cooler have the same entering air temperature (35˚F) and use the same refrigerant (R404A), the walk-in cooler's evaporator coil temperature would be 25˚F. The suction pressure would be 62.7 psig with a TD of 10˚F.
• Reach-in Cooler: A reach-in cooler, with a coil temperature of 15˚F and a 20˚F TD, would have a suction pressure of 49 psig.

Although both coolers maintain the same case temperature, they operate with different RH levels and slightly different suction pressures due to varying TD values.

Many DIYers and HVAC technicians rely solely on refrigerant pressure to charge a system, which may achieve the desired box temperature but fails to consider proper system operation, product longevity, and profitability—knowledge that sets average HVAC technicians from true HVAC Experts apart.