Rethinking Evaporator Hot Gas Defrost Times

Hot gas defrost is one of those processes in an ammonia refrigeration system that can easily become a “set it and forget it” operation. An evaporator may have been programmed for a 30-minute hot gas defrost years ago, and as long as the coil stays clear, nobody questions it. But just because a defrost cycle works does not necessarily mean it is operating efficiently.

The challenge with that mindset is that a defrost cycle can be effective without necessarily being efficient. Just because the coil is free of frost at the end of a 30-minute hot gas defrost does not mean it actually needed 30 minutes of hot gas. The frost may have been completely removed much earlier in the cycle, with the remaining time simply adding unnecessary heat into the evaporator and refrigerated space.

We are specifically talking about the period when the hot gas defrost solenoid is energized and hot gas is actively being supplied to the evaporator. This does not include pump-down time, equalization, fan delay, or the other stages that may be incorporated into the complete defrost sequence. Those other sequence steps will be discussed in future blogs.

Most Common Hot Defrost Adjustments

When making adjustments to hot gas defrost operation, two of the most common immediate changes you can make before doing deeper troubleshooting involve either increasing or decreasing the defrost back-pressure regulator (BPR) setting. Both adjustments can have their place, depending on what is actually happening within the evaporator during defrost.

Increasing defrost pressure can provide a higher hot gas temperature and may help overcome certain defrosting problems, while reducing defrost pressure can help limit unnecessary heat input and improve overall defrost efficiency. The key is understanding why the adjustment is being made instead of simply changing pressure until the evaporator appears to defrost properly.

In the sections below, we’re going to break down why both approaches can be appropriate, what conditions may justify each adjustment, and why defrost pressure should be evaluated along with hot gas supply pressure, pressure differential, condensate drainage, and defrost duration.

More Heat Isn't Always Better

The purpose of hot gas defrost is relatively simple: provide enough heat to the evaporator to melt the accumulated frost and allow that moisture to properly drain away. Once enough heat has been provided to accomplish that task, additional heat has diminishing benefits. As hot gas temperature increases above what is necessary to melt the frost, more energy can be transferred into the refrigerated space instead of being used productively for defrost.

In other words:

The goal isn't to make the evaporator as hot as possible. The goal is to provide enough energy to complete the defrost effectively.

A practical minimum for effective defrosting under normal conditions is approximately +50°F hot gas temperature, corresponding to approximately a 75 psig defrost regulator setting for ammonia. To melt ice at 32°F back into liquid water, you generally need roughly a 18–20°F temperature difference to provide effective heat transfer during the defrost process. When hot gas temperature is increased from approximately +50°F to +90°F, heat transfer rate increases but the percentage of energy lost to the refrigerated space also increases.

That additional heat eventually has to be removed again by the refrigeration system.

So, while increasing hot gas pressure or temperature may appear to improve a problematic defrost, it can sometimes be masking another problem rather than correcting it.

Pressure Differential Matters

Hot gas must have enough pressure differential to move through the evaporator and push condensed refrigerant out of the coil. Approximately a 15–20 psig pressure differential between the hot gas supply and the defrost BPR is usually sufficient. You should always reference the manufacturer’s information first to determine whether there are any recommended differential pressure requirements for the evaporator or defrost system.

It can be easy to assume that raising the BPR setting will improve defrost by increasing the saturated defrost temperature, but there is another side to that adjustment: as the BPR setting increases, the available pressure differential across the defrost circuit decreases. This is why lowering the defrost BPR setting may be recommended when it is already at 75 psig or higher. As the BPR setting increases, the available pressure differential decreases, which can make it harder to move condensate out of the evaporator.

If that differential becomes too small, condensed liquid refrigerant may begin accumulating in the evaporator.

This can occur when:

• Hot gas supply pressure becomes too low

• Defrost regulator pressure is set too high

• Condensate cannot properly drain from the evaporator

• Defrost piping is improperly designed or installed

• Liquid becomes trapped within portions of the coil

  
  

Once condensed refrigerant begins accumulating and becoming subcooled, heat transfer available for melting frost is reduced. This is why simply lowering pressure in the name of energy efficiency isn't necessarily the answer either. An efficient defrost requires both adequate heat and proper condensate removal.

Longer Defrost Times May Be Hiding a Problem

If an evaporator genuinely requires an unusually long period of hot gas to clear the coil, shortening the timer may not be the first thing that needs to happen. Instead, determine why the longer defrost is required.

Potential issues can include:

• Improperly trapped defrost piping

• Poor condensate drainage

• Ice accumulation in the drain pan

• Incorrect control valve settings

• Insufficient hot gas supply pressure

• Improper pressure differential

• Coil circuiting or evaporator design issues

• Excessive moisture infiltration into the refrigerated space

• Defrost frequency that does not match actual frost accumulation

  
  

Increasing pressure or increasing defrost duration can sometimes overcome these problems temporarily. But that doesn't necessarily correct the underlying issue. How Long Does Hot Gas Really Need to Be On? Approximately 8–10 minutes of active hot gas flow is typically sufficient to melt frost from evaporator tubes and fins. Yet it is still common to find industrial refrigeration systems with the active hot gas portion of defrost programmed for 20–30 minutes or longer.

That doesn't mean every evaporator should immediately be changed to a 10-minute defrost.

Evaporator design, room conditions, frost accumulation, piping configuration, control sequences, hot gas pressure, condensate drainage, and manufacturer recommendations all matter.

More Frequent Defrosting Isn't Always More Efficient Either

Another interesting consideration is frost thickness. It seems logical to defrost frequently so there is as little frost as possible on the evaporator. However, every defrost cycle introduces heat into the refrigerated space. Allowing additional frost to accumulate before initiating defrost can improve the percentage of defrost energy actually used to melt frost.

That doesn't mean operators should intentionally allow coils to become heavily iced.

Excessive frost reduces airflow and evaporator performance. But it does reinforce an important point: There is an optimal balance between defrosting too frequently and waiting too long.

Defrost should occur because the evaporator needs it, not simply because a timer has always initiated one at that interval. One way to get a more accurate picture of each evaporator’s actual frost accumulation is to use an ice or frost detection sensor on the coil. Rather than initiating defrosts strictly on a fixed schedule, regardless of whether the evaporator actually needs it, the sensor can help trigger the defrost sequence based on real operating conditions.

This can reduce unnecessary defrost cycles on coils that have little frost accumulation while still ensuring heavily loaded evaporators receive defrost when needed. In that sense, the system is responding to the condition of the evaporator instead of simply relying on a timer.

That said, sensor-based defrost controls still need to be properly selected, installed, tested, and maintained. The sensor location, reliability, failure mode, and control logic all matter. A poorly placed or failed sensor could create just as many problems as an overly aggressive time-based schedule.

Don't Change Everything at Once

When evaluating defrost efficiency, make controlled changes and document the results.

For example, if an evaporator currently receives 30 minutes of active hot gas, you wouldn't necessarily jump directly to 10 minutes. Instead, establish the current baseline and determine when the frost is actually being removed.

Adjustments could then be evaluated incrementally while monitoring:

• Remaining frost or ice

• Drain pan condition

• Coil temperatures

• Room temperature

• Suction pressure after returning to refrigeration

• Hot gas and defrost pressures

• Defrost frequency

• Compressor loading and system response

Final Thoughts

Hot gas defrost shouldn't simply be treated as an on/off function that either works or doesn't work. Pressure, temperature, duration, frost accumulation, condensate drainage, and defrost frequency all work together. Increasing hot gas pressure or extending a defrost timer can make a problem disappear visually, but it may come at the expense of refrigeration efficiency and may conceal deficiencies elsewhere in the defrost system.

The goal isn't to achieve the shortest possible defrost. The goal is to achieve the shortest effective defrost that consistently removes the required frost and allows the evaporator to return to refrigeration without creating additional operating problems. Most importantly, changes to automated refrigeration sequences should follow your facility's established procedures, manufacturer requirements, and applicable Management of Change (MOC) process when required.

Stay tuned to the MySafety blogs. Reach out to us if you have additional questions or concerns. Our team will be pleased to assist you.

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Rethinking Evaporator Hot Gas Defrost Times