Many of today’s companies that utilize liquid nitrogen (LIN) as a refrigerant fight an uphill battle to retain profits on the products they freeze. The four main contributors to this are the never-ending rising cost of nitrogen, waste associated with the freezing equipment, storage tank pressure and the process itself.
Here’s how to identify common issues with tank pressure and freezers, and then retrofit energy efficient features.
Correct tank pressure
In most cases, the tank pressure (lower and upper set points) are set by the supplier to ensure that there are no supply interruptions during the production schedule. However, there have been many cases in which the tank pressure has been set much higher than it should. There aren’t many warning signs of this condition because there are no supply interruptions. However, available British thermal units (BTUs) in each pound of LIN can be lost if the tank pressure is set too high.
LIN at atmosphere pressure is approximately -320°F. That temperature starts to rise as it is placed in a tank and pressurized; the higher the tank pressure, the warmer the nitrogen.
Inefficiencies are related to several areas of the freezer. Not all freezers need to be replaced, but critical upgrades can yield huge results.
The actual carcass—Is the freezer insulation saturated? Wet insulation is worse than no insulation at all.
Heat transfer components—Out-of-date components can rob you of available heat removal opportunities.
Control of the expanding gas as the LIN enters the freezer—Regardless if the freezer is designed to present the nitrogen to the product in liquid form (latent heat) or as a cold gas (sensible heat), it still enters the freezer in liquid form. The challenge comes when the LIN expands to a gas. Keep in mind that when this happens, there is an expansion ratio in volume of nearly 700 times. Retaining the gas within the confines of the freezer is critical, as it relates to efficiency and safety, such as the lowering of O2 levels, not being able to see where you are walking and the possibility of freezing any residual water still on the floor from cleanup.
The most critical gas control period is during actual production because this happens over the course of a full production day, whether that be 8 hours or 16 hours.
Any type of nitrogen freezer should operate under positive pressure. This can be noted by observing the freezer while it is in production. There should be a small amount of vapor “rolling” out of the entrance and discharge sections. This is visual confirmation that you are only paying to freeze or cool your product, not the room air, which is often being pulled into the freezer by poor gas flow controls.
First, the physics of a cool down. Each freezer, no matter the size, has its own identity regarding fixed and cooldown losses. That is, each time a freezer is cooled down at the beginning of a production run, it will take a certain amount of nitrogen to cool all the stainless-steel mass down to the set point temperature (cool down losses). Once the set point temperature has been reached, it will take another certain amount of nitrogen to keep that freezer at that temperature throughout the full production day (fixed losses).
Many freezer operators will start the cooldown process each morning at the same time. But, have they communicated with production and maintenance to ensure the plant will be ready to freeze at the appointed time, or will the freezer sit at set point temperature consuming nitrogen without product?
Quality control—Many topics should be covered with that specific team.
Current available mechanical refrigeration—Are there ways to utilize what is currently available in conjunction with the nitrogen freezing system?
Some applications warrant a few seconds of LIN, locking in the moisture as it freezes, thereby gaining some yield.
Has every BTU been extracted from the exhaust stream before it is sent back to the atmosphere?
It is always a battle to keep the price of nitrogen to a minimum. However, there are many ways to reclaim the margins on your product. Contact a legitimate expert to come in and perform an audit. Most companies would be shocked to find out how many dollars are flowing out the exhaust stack.