Refrigeration systems in freeze drying provide the cooling capacity to freeze products directly upon the freeze dryer shelves, and to provide cooling of the “condenser” to provide a receptacle for the water that is subliming from the frozen drying product.
The refrigeration systems can take the form of tiny fractional horse power “canned” type, accessible hermetic, open type compressors with shafts and external motors, screw compressors, and liquid nitrogen powered. By far the best overall system from simplicity and low maintenance is one powered by liquid nitrogen.
Whichever system is used the system must have the refrigeration power to allow products to be frozen to nearly -55 °C and condenser temperatures to -75 ºC, which are by most standards very cold indeed. The mechanical compressor refrigeration systems operate at these temperatures by using refrigerants like R507 or R404A, and are tuned to operate at a suction pressure of between 22-29 inches of vacuum with a discharge pressure of 250 to 275 psig. What this means ultimately is that these compressors operate in ranges of pressure that are very stressful and cause excessive wear to compressor components. In the piston type systems this wear can cause even the most robust designs from Copeland, Bitzer, Tecumseh, and Carlyle to have a useful lifetime of about 5 years, if and only if they are maintained on a daily, weekly, monthly, and yearly basis by refrigeration experts. The slightest suction leak in these systems causes a multiplicity of stresses on the systems. Briefly some of these stresses are listed as follows: high head pressure, lubrication acidification, internal corrosion, and valve breakage. As a result of these insidious and difficult to diagnose, time consuming sorts of problems, batches can be lost, and downtime increased, and production slowdown is inevitable. It is actually amazing that the technology and engineering that goes into the production of mechanical compressors enables them to function in such adverse conditions for as long as they do. It is akin to operating your car in 1st gear while traveling interstate highways at normal highway speeds! This is ruinous to the engine, and would require much attention to maintenance if operated in such a way.
Liquid nitrogen powered systems are much simpler, an order if magnitude quieter, and very reliable. In fact it has been stated that the maintenance cost difference between LN2 systems and mechanical systems easily justify the higher cost of liquid nitrogen systems in comparison. Mechanical systems cost less to purchase initially. Mechanical systems cost less to purchase, and operate from the standpoint of utility costs. Mechanical systems have greater need for infrastructure such as cooling towers and maintenance, experience higher electrical energy costs per unit of operational time, require larger more expensive diesel operated backup generators, are installed with many more internal controls and devices than a LN2 system. If one is choosing a refrigeration system and chooses a mechanical system, it is best to consider screw compressors VS. piston type compressors. Screw compressors require fewer controls, stand up to adverse operating conditions better than piston compressors, and cost more than piston compressors initially. Screw compressors stand up to abuses such as accidental slugs of liquid refrigerant, they need fewer oil changes, and have more cooling capacity at low temperature than piston compressors. Screw compressors are also quieter in operation than piston compressors.
Applewood Scientific is well placed to assist you in your initial choices, detailing utility requirements, infrastructure, and process related issues dealing with your current refrigeration systems. We can also assist you in conversion of mechanical systems to LN2, providing you with the resources needed to accomplish this transformation.