COMMON TERMS USED IN FOOD CHILLING AND FREEZING
BTUs: (British Thermal Units)
A standardized energy (heat) measurement. A BTU is defined as the amount of energy required to heat one pound of water 1 degree Fahrenheit. Example: Assume you are a pork processor and you want to chill a pound of meat from 90°F down to 35°F. Your change in temperature is 55°F, but in order to effect that temperature change, you need to remove 45 BTUs of heat from the meat. (Want to know how to make that calculation or how to actually accomplish this heat removal?) CONTACT US!
When we refrigerate an object we are removing heat from it. Heat transfers from the warmer object (the food product) to the colder object (the cryogen or mechanically chilled air) until both objects reach the same temperature.
This is the number of BTU’s of heat a refrigeration source can remove, expressed as BTUs per pound of cryogen. This number can be expressed as a maximum refrigeration value, or as the actual refrigeration value that can be expected in a production environment. Example: One pound of liquid CO2 has a maximum value of 120 BTUs of refrigeration (see Latent Heat of Vaporization from gas properties chart); but we know from practical experience that a CSE designed freezer will deliver about 100 BTUs per pound of liquid CO2. This becomes the basis for calculating your cryogen usage and that portion of your freezing cost. Maximum values are always the same, but actual values vary based on freezer design and freezing process parameters. (At CSE, we can help you project actual refrigeration values and costs.) CONTACT US!
This is the actual refrigeration value of a cryogen as it performs in a specific freezer. Regardless of whether a freezer is mechanical or cryogenic, it will operate at less than 100% heat transfer efficiency (which only occurs in a laboratory setting). This is because there are many variables that impact its performance and “rob” BTUs. These variables include undesirable (but avoidable) elements such as warm air infiltration into the freezer, less than optimum belt loading, and improper operating procedures. They also include unavoidable refrigeration losses due to the steady state losses of keeping the freezer at operating temperature, freezer cool down procedures or less than optimum storage conditions of the cryogenic gases.
HEAT TRANSFER RATE
This is the speed at which a freezer can remove heat. This information is very specific to both the type of freezer, the cryogen and the food product being processed; it is the basis of calculating what kind and size of freezer is required.
HEAT TRANSFER MODE
Describes how the actual heat transfer occurs. This is a major consideration in freezer design.
There are three modes of heat transfer:
Vapor-to-solid heat transfer – This is cold vapor being passed over a warm solid food product (could be N2, CO2, or mechanical refrigeration). This is sometimes referred to as “vapor-stripping”, because BTUs are being “stripped” from the cold vapor.
Solid-to-solid heat transfer – This is solid dry ice particles (CO2 snow) being directed at the food product (CO2 only)
Liquid to solid heat transfer – Liquid nitrogen can be sprayed on the product, or the product can be placed in a liquid nitrogen bath (LN2 only)
To get a sense of the impact of heat transfer rates, envision this:
- Putting your hand in a bucket that contained 40°F air (vapor-to-solid).
- Burying your hand in a bucket that contained 40°F sand (solid-to-solid).
- Plunging your hand into a bucket of 40°F water (liquid-to-solid).
Obviously, 40°F water would be very unpleasant immediately, the sand wouldn’t be far behind, and after a couple of minutes, the 40°F air would be equally uncomfortable. All three modes transfer heat out of your hand, and the temperature in the bucket is the same, but the heat is transferred at different rates. CONTACT US!