Denis, Thanks for your reply. In your reply you pose the question “It is correct that in most part of the world +32°C data is the most used design value; however should we leave aside more extreme conditions?” In my opinion, this dilemma could be resolved by looking at climate data more closely; and considering which refrigerator characteristics are affected by average daily temperatures, and which are affected by peak temperatures. For example, for direct drive system and ice pack carriers the number of days of autonomy are primarily dependent on the average daily temperature. For a battery powered system the ice making capabilities are also mainly dependent on the average daily temperature. For both direct drive and battery powered the energy use is also dependent on average daily temperature.
Since it is extremely rare to experience average temperatures which are above 32 deg C we have the following suggestions. If the installation is to be placed in a location where daytime temperatures approach 43 deg C, I suggest using the “Stable Run Test” data at 43 deg C, to make sure the vaccines stay at an acceptable temperature. Then use the 32 deg C data to indicate the number of days of autonomy for a direct drive system, the ice making capabilities for a battery powered system, and their energy consumption. This method has a built-in safety factor. When the temperature extremes are experienced, the sun will be bright and the need for a large number of days of autonomy will be decreased. For a battery powered system more energy will be available for ice making.
Requiring ice making tests and the measurement of autonomy time at 43 deg C will have a negative impact on performance. As a consequence of the way a compressor powered system incorporating a capillary tube work, having to make ice at 43 deg C will lower the evaporator temperature at cooler temperatures. This would result in decreased efficiency, increased system costs, and on some models, greater temperature swings. In a direct drive system, efficiency would be reduced at 32 deg C and below.
I propose new testing procedures to assure that vaccines are stored safely when ambient daytime temperatures approach 43 deg C. They would be similar to the day/night test already in use. Vaccines would be kept at 32 deg for 12 hours and then 43 deg C for 12 hours. The MKT temperature could then be used to verify that vaccines were safely stored.
Attached is some temperature data I extracted from a report you coauthored, “Use of Cool Water Packs to Prevent Freezing During Vaccine Transportation at the Country Level”. The graphs are for a location in Myanmar and Zimbabwe. At both locations peak temperatures are around 40 deg C, however, average daily temperature are about 32 deg C or below. Temperature drops into the mid 20’s are common where temperatures peaks near 43 deg C. This could easily be verified by a statistical search of hot spots on the NASA website or data compiled on the BBC weather website.
Another related point I would like to address is we often get RFQ’s for projects in the tropics where hot zone PQS rating is required, however some of these areas are actually in a temperate zone. I have a feeling in these instances climatic data was never examined. Here are some examples of some equatorial locations that are actually in a temperate cool climate zone:
Location: Average High (Hottest Month) Average Low (Hottest Month) Record High
Kinshasa, Congo 32 deg C 22 deg C 36 deg C
Manaus, Brazil 32 deg C 24 deg C 38 deg C
Port Moresby,New Guinea 32 deg C 24 deg C 37 deg C
Monrovia, Liberia 31 deg C 23 deg C 34 deg C
Quito, Ecuador 23 deg C 7 deg C 30 deg C
In summary here are my suggestions:
- Use records of ambient temperatures to determine climate zones
- If daytime temperatures can approach 43 deg C use 43 deg C stable run test results to determine if vaccines will be safely stored. Use 32 deg C test data to determine energy consumption, autonomy time, ice making capabilities and storage times for ice pack carries.
- Incorporate a day/night test for hot climates where daytime temperatures are 43 deg C and nighttime temperatures are 32 deg C. A more realistic test such as this could improve system costs and performance.