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Closed Loop Chemical Selection Guide


You have two main considerations when determining your closed loop chemicals:
  1. Corrosion Protection
  2. Bacteria Prevention
First corrosion, there are two popular closed loop corrosion inhibitor chemicals:
  1. Nitrite based
  2. Molybdate based
When determining which chemical to use you must consider:
  1. How valuable your closed system is
  2. Cost of down time
  3. Water system temperature
  4. System skin temperature
  5. Heat transfer
  6. Water loss
  7. System metals
  8. Does the product need to be food grade certified
Corrosion Inhibitor Selection
The best closed loop corrosion protection chemical is a straight or molybdate based chemical. Ideally, this would be the best chemical for most closed loop systems. However due to the cost of molybdate it is often difficult to cost justify. In some cases, the cost of downtown or the value of the system easily justifies spending the extra money on molybdate. You need to make this call. Sodium Nitrite based solutions are the most popular corrosion inhibitors in a closed loop system. The only issue that may be a problem with nitrite since nitrite is a food source for bacteria development, as discussed below. When feeding either chemical, both molybdate and nitrite are initial charged to the system and fed as needed based upon system water loss.

Closed Loop Temperature
Systems below 45F, typically are resistant to bacteria growth, while systems above 45F is more prone to bacteria development. So the system temperature needs to be considered when selecting your chemical. Nitrite is a food source for bacteria development within the closed loop system, while molybdate is not. If your system is above 45F, you still may use a nitrite based corrosion inhibitor, but will need to implement the use of a non oxidizer chemical such as isothiazolin or gluteraldahyde .

Bacteria Chemical Selection
Closed loop chiller systems are prone to bacteria growth, like anaerobic or aerobic bacteria. Bacteria may lead to premature system failures, loss of water flow, loss of heat transfer, leaks as a result of under deposit corrosion, system erosion, and unexpected downtown. So depending on how important your system is, you may choose to monitor bacteria within your water closed loop water systems with bacteria dip slides. Solutions to bacteria include:
  1. The use of a non oxidizer chemical
  2. Use of Propylene Glycol above 20%
The most popular non-oxidizer chemicals are gluteraldahyde and isothiazolin. These chemicals are fed based on your bacteria dipslide readings, that is when it exceeds 10^3. In theory, the non-oxidizers are alternatively fed to prevent the bacteria from becoming resistant to the chemical. Also, keep in mind isothiazolin is a sensitizer and some people have allergic reactions when exposed to the isothiazolin vapor.

Some closed loop systems, such as small cooling loops in medical equipment, are used in the field and testing and when monitoring of the corrosion inhibitor and bacteria development are not possible. In these systems, you may consider using a corrosion inhibited propylene glycol solution above a 20% solution. Propylene Glycol is resistant to bacteria growth and the inhibitor will minimize the chance of corrosion.

Food Grade Corrosion Inhibitor
If you have a system that requires corrosion protection and food grade certification, DPK is a good choice. Molybdate and Nitrite based chemicals are far superior in corrosion protection versus DPK, but the latter will do a good job minimizing corrosion.
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