Operational Efficiency: Condenser Drain Lines

Operational Efficiency: Condenser Drain Lines

By: Jayson Noetzelman

 

     Carlson and Stewart Refrigeration would like to take some time to address an often-ignored problem in industrial refrigeration systems. The problem is a lack of knowledge about pressure requirements to successfully operate a spring closed check valve. Spring closed check valves are often used without issue in refrigeration systems but can be particularly problematic at the outlet of evaporative condensers. Unfortunately, those little check valves we install to prevent liquid migration can have a big impact on your system. This happens because to overcome the spring on a typical spring closed check valve, we need anywhere from .25-2 psi. Because the high-pressure receiver, where the liquid is draining into, and the condenser are equalized, we can only generate pressure by means of potential energy. To create that potential energy, we must stack liquid ammonia on the check valve. How much stacked liquid height do we need to overcome the spring? If our system is perfectly balanced, well equalized, and has a low pressure drop through the condenser and nearby valves we might estimate just 1 PSI for the system + the rated psi loss through the check valve. If that check valve is a commonly available 2 psi drop valve, we would have 3 total PSI of pressure drop through the condenser, and thus need to stack 3 psi worth of liquid ammonia on the check valve. To find how many feet of liquid is represented by 3 psi we would utilize the formula:

     When we plug in the appropriate values for liquid ammonia, we find we would need about 3.9 feet of liquid stacked for EACH psi required! This means we might need as much as 12 feet from our condenser outlet to the top of our check valve even on a well equalized system. Does your facility have twelve foot of vertical drop from the condenser outlet to the check valve? If not, you could be backing up liquid into the condenser coils in order to overcome the spring resistance in the check valve. That means you are losing condenser surface area for vapor heat exchange and therefore condenser capacity.

     The solution is, not surprisingly, lowering your pressure drop. Carlson and Stewart Refrigeration typically considers installing a widely available low-pressure activation check valve, which requires only .25 psi to open. This could save you nearly seven feet of required distance alone. Also consider installing that check valve as low as you can before the P-Trap (if your lines are trapped). Let’s look at an example to drive the point home. If we had 3 psi pressure drop in an imaginary ammonia refrigeration system, we would need approximately 12 feet of vertical distance from condenser outlet to check valve. Let’s imagine we have a system with just 10 feet of vertical drop distance. If we need to stack two additional feet of liquid to overcome the check valve our system is going to keep stacking up liquid into the condenser coils. Condenser coils are typically long horizontally but only 3-6 feet tall vertically. This means stacking a couple feet of liquid could cost us 33-66% of our condensing capacity. A system backing up liquid into the condenser will often experience a surge of liquid back to the receiver causing the receiver level to rise rapidly in a rather short time period. This can be a good indicator you are stacking liquid in your condenser. By replacing a 2 psi drop required check valve in our imaginary system with a .25 psi light spring check valve we could reduce the required liquid height to roughly 5 feet, and therefore well below our 10 available feet.  Stacking liquid not only wastes money to run the condenser fans but more importantly misuses costly condensing capacity. If you feel your condensers just aren’t meeting their rated capacity, Carlson and Stewart would recommend analyzing your system’s condenser equalization and draining system before installing costly new condensers.

     The problem isn’t always easy to see, but it is easy to see how costly a seemingly small mistake like an improperly installed check valve can be on your ammonia refrigeration system. Luckily, with the availability of low pressure drop check valves a solution is obtainable. Carlson & Stewart Refrigeration (CSR) operates out of Minnesota and South Dakota and serves customers in the surrounding states as well. If you are located in these areas and feel you might be due for some changes to get your ammonia system running effectively and efficiently, give us a call and we would be glad to help.