I had attempted to reply to this thread earlier, but was thwarted by my IT department’s automated insistence that this forum is not work-related… so I’m trying again,
Temperature Excursions
The acceptability of temperature excursions is based on the impact on the materials stored within the unit. I would not encourage you to take the advice of someone who doesn’t know that impact to judge the excursions as acceptable. Look to stability studies for the determination of the impact on the materials stored within the freezer.
There are a number of things that you can do to improve the temperature profile and response so you don’t need to worry about digging through stability data. I’ll go into some of them here. Note that most of these recommendations assume that you’re using the more common compressor/evaporator type refrigeration system, and not a cold box, which relies on cold walls to keep contents at temp.
Mechanical controls are fairly common on chambers. They are inexpensive, but less reliable. I’ve found that replacing mechanical controllers with digital controllers generally improves chamber performance. You can also look at the offset. The offset is a value that is used to reduce compressor cycles. Fewer cycles keep the compressor running longer. Decreasing the offset will narrow the range in which the compressor cycling is suppressed by the control system, which will allow for a more rapid response, by allowing the compressor to cycle more frequently. This will wear out the compressor more quickly, but is sometimes justified based on the criticality of the materials within the chamber.
I’d also suggest looking at defrost cycles and termination, which may be driving some of the high temperature values. Your evaporators will periodically go into a defrost cycle, which is terminated either on time or temperature. Changing the defrost parameters may give you a better temperature response while still preventing frost build-up on the evaporators. You have to be careful that the defrost is still adequate, however, or the gradual build-up of frost will drive the chamber out of specification and could potentially damage the refrigeration equipment. You may also want to look to modify control to turn off evaporator fans while a defrost cycle is active to prevent the fans from blowing warmer air throughout the chamber during a defrost.
Look at how the probes and product are located within the chamber. It is not good practice to put product directly in front of evaporator fans, as the air coming off them is generally much colder. If you can quarantine an area where your high or low temperatures are located and prevent it from being used, that is a generally accepted practice. That may allow you to cut out the areas where your temperatures are routinely outside of specifications. I would recommend that you physically obstruct the quarantined area with something that is air-permeable (a wire cage, for example), as it is much more difficult for people to ignore than signs and procedures. In high-criticality applications, you may need to remap after installation of a barrier to ensure that you haven’t adversely impacted the chamber’s temperature profile.
Look at where the control probe is located. It should similarly not be located directly in the air stream from the evaporators, or it will lead to much more exaggerated temperature swings and excessive compressor cycling.
I disagree with performing three temperature runs, unless you’re changing conditions, such as control configuration or loading patterns, in between. The data from three consecutive runs is generally similar enough that you won’t gain anything from having the three replicate runs. As long as your runs are long enough to ensure several compressor cycles. 24 hours seems to be widely accepted, but you may be able to make the case for 8- or 12-hour periods as well.
The thermocouple must be placed in the air for mapping to demonstrate the air temperature response. Do not place the thermocouple in contact with anything, such as product or chamber walls, as it will skew results. If you want to monitor and alarm the chamber with a probe in glycol to dampen the effects of door openings to eliminate nuisance alarms, that is generally viewed as acceptable. It is not generally accepted practice to map chambers with probes in fluid, such as alcohol or glycol, for the same reason – it dampens the temperature response.
Since you live within 3 hours drive of me, I may be able to recommend some refrigeration contractors who can help with your problem that are within your range. Contact me if you’re interested.
If you can send your temperature data and mapping configuration, redacted for proprietary and confidential information (rich.paules@roche.com), I may be able to provide some better advice based on your specific situation. If I think of anything else, I’ll try to get back here to update this.