Noncondensables are gases such as hydrogen and other foreign gases that will not condense into a liquid. Noncondensables can cause serious problems for your vapor absorption chiller (i.e. absorber), but the problem can be difficult to diagnose.
In this article, you’ll learn how to determine if noncondensables are present in your absorption chiller. But first, let’s explore how noncondensables end up in your absorber, what will happen if you ignore the problem, and how noncondensables can cause your absorber to malfunction, lose capacity, or lead to an overall loss of efficiency for the absorption chiller.
What Causes Noncondensables in an Absorption Chiller
One of the most common causes of noncondensables is improper evacuation of the absorber during a new installation or repair. Sloppy procedures, faulty tools, or improper parts essentially allow noncondensables to enter the system. Over time, noncondensables can also work their way into the absorber through the normal processing of the working solutions. Noncondensables can also find their way into the absorption chiller through leaks in the various different system components or parts.
Consequences of Noncondensables in an Absorber
When noncondensables are present in an absorption chiller they result in a loss of capacity of the absorption chiller. Noncondensables can also lead to internal corrosion, crystallization, or depletion of the inhibitor. Before you know it, this can lead to more lasting damage and much more expensive repairs (e.g. plugged or choked heat exchangers, corroded tubes, pitted tubes, tube leaks, etc.). The more noncondensables are allowed to build up, the greater the loss of the absorption chiller’s efficiency or capacity, and the greater the potential damage (including significantly shortening the life of the absorption chiller).
The capacity of an absorption chiller is heavily dependent on the temperature at which the water in the evaporator section boils, and is evaporated. In turn, the temperature of the refrigerant water depends on the vapor pressure in the absorber section of the chiller.
Noncondensables foul up this process by increasing the vapor pressure in the absorber. This leads to an increase in the evaporator temperature which, in turn, leads to an increase in the temperature of leaving chilled water. A higher temperature of leaving chilled water, in this case, indicates a greater volume of noncondensables in the absorption unit.
Along with a reduction in capacity, another clue that noncondensables are present in an absorption chiller is that the temperature controller will indicate the need for increased input. This can lead to maxing out the solution concentrations. This, in turn, can lead to crystallization in the heat exchanger, and, if neglected, increased damage, leaks, and costly repairs.
How to Check for Noncondensables in a Vapor Absorption Chiller (i.e. Diagnosing the Cause of the Absorber’s Loss of Capacity or Efficiency)
The following are the steps to take to determine if noncondensables are present in the absorption chiller:
Figure 1
- First, take a sample of the dilute solution to determine its concentration (see Figure 1 = 59.2°).
- Second, measure the temperature of the absorber (see Figure 1 = 105°F).
- Third, look to the equilibrium chart below (see Figure 1), and locate the point of intersection between the dilute solution concentration (in Step 1), and the absorber temperature (in Step 2).
- Fourth, locate the corresponding refrigerant temperature on the equilibrium chart by looking all the way to the left of the point of intersection you identified in Step 3 (see Figure 1 = 38°).
- Fifth, measure the actual refrigerant temperature.
- Sixth, compare the actual refrigerant temperature identified in Step 5 with the refrigerant temperature you located on the equilibrium chart (in Step 4).
- Seventh, subtract the refrigerant temperature referenced on the equilibrium chart from the actual refrigerant temperature. Determine if the actual refrigerant temperature is more than 6°F greater than refrigerant temperature referenced on the equilibrium chart. If so, than noncondensables are present.
This is the most accurate way to determine if noncondensables are present in the absorption chiller. Do not be fooled by the apparent lack of noncondensables from the exhaust of the purge pump. The purge pump is not always an accurate indicator, especially if the pump itself is faulty.
Does the Absorption Chiller Have a Leak?
Once you have determined that noncondensables are indeed present, the next step is to ensure that the purge unit is operating properly. If the purge unit is operating properly, then you may indeed have a leak in your absorption chiller.
In fact, besides a leak, the only other possible explanation is that the absorber is generating excessive noncondensables in the form of hydrogen (or other noncondensables gases). This would be a good time to determine if the noncondensables present are in fact hydrogen, but it would also be a good time to call for some expert help.
Insider Tips
- An absorption chiller will never be completely free of noncondensables. In fact, it is virtually certain that some noncondensables will be generated during the normal functioning of an absorption chiller unit. The key is to keep these noncondensables to a bare minimum.
- Do not neglect to take a solution sample every year and send it to the lab for chemical analysis. These recommendations are critical to ensuring the inhibitors in the absorber are correct, which is key to preventing the formation of gases, iron oxide, and premature corrosion of the absorber’s shell—all of which will, ultimately, shorten the life of the absorption chiller.
Safety Tip
Never ever, ever attempt to start the motor of a seized vacuum pump by moving the belt with your hand or fingers! (If you get a chance sometime, ask me how I learned this lesson.)
Defining Terms
Noncondensables—gases such as oxygen and hydrogen that will not condense into a liquid.