One might ask, “How much harm can a small section of poorly insulated or un-insulated pipe cause when the major portion of the system is vacuum insulated?” The answer is, “plenty of harm.” The loss is not immediately visible nor is it significant for one day, but over time that loss can add up to thousands of dollars. (see Product Loss Comparison chart).
Due to the extreme temperature difference between liquid nitrogen and ambient air a large amount of heat will transfer through a very short section of un-insulated or poorly-insulated pipe or component very quickly.
Even a short section of exposed pipe can have a substantial impact on the entire system. Increased heat leak at any point in the system can cause two-phase fluid that increases pressure drop, causing irregular flow of liquid, which reduces the overall flow rate. Two-phase flow will create significantly higher pressure drops through the pipe system, irregular liquid delivery, results in warmer liquid at the cryogen use point
and shortens the life of valve seats and other components within the system. Let’s look at two examples:
Let’s assume we have a 100-foot run of vacuum jacketed pipe (VJP) with a 2-foot connection of foam-insulated pipe.
Vacuum Jacketed Pipe (1.0 in. dia.) typically transfers about .47 BTU/hr/ft Total Heat Leak of the 100-ft run of VJP is 100 x .47 = 47 BTU/hr
Foam Insulated Copper Pipe typically transfers about 20 BTU/hr /ft Heat leak of a 2-ft section of foam insulated copper pipe is 2 x 20 = 40 BTU/hr
The 2-foot section of foam insulated copper pipe is responsible for 40 / (40+47) 46% of the total heat transferred into the pipe.
Total heat leak for the 100-foot run of fully vacuum insulated pipe is 47 BTU / Hour
The heat leak for the 100-foot vjp system plus the 2-foot foam insulated copper pipe is 87 BTU/hr
The consequences escalate with bare, un-insulated pipe.
Heat leak of a 2-foot section of bare copper pipe is 2 x 400 = 800 BTU/hr
The two-foot section of bare copper pipe is therefore responsible for 800 / (800+47) 95% of the total heat transferred into the pipe.
Needless to say that with energy costs rising, any new or upgraded liquid nitrogen system should be designed with a complete system perspective in mind in order to obtain the most efficient system for optimum performance and lowest maintenance cost over the life of the system.