How to check condenser approach in water cooled chiller | Approach Temperature

Background:

A water-cooled chiller will have two heat exchangers namely the evaporator and condenser. The evaporator circuit is a chilled water loop where the heat from inside conditioned space will get exchanged to refrigerant through chilled water. A chiller condenser circuit is another loop where the heat from the refrigerant is being rejected into the atmosphere through cold water from the cooling tower. Chillers' efficiency will be on the higher side if both loops function effectively. 

The evaporator circuit is a closed-loop circuit where the same clean water gets recirculated continuously. So there will be no chance for entry of any foreign material to enter into the loop and choke. But in the case of the chiller condenser, it's an open-loop circuit and hence high chances for deposits of soft organics or hard minerals like iron, manganese, or calcium carbonate (calcite) to happen. This will clog the condenser tubes in the long run. These minerals even will form a hard layer inside condenser tubes which reduces the effective functioning of a condenser. Since a chiller condenser is nothing but a heat exchanger, clogging or fouling of tubes reduces effective heat transfer area and reduces the condensing capacity drastically.

Approach Temperature & Condenser Approach:  

The performance of a condenser can be evaluated with the help of the condenser approach. Before knowing about the condenser approach let's first understand what's approach temperature mean. That's the temperature between the leaving process fluid/liquid to that of entering utility.

A condenser approach temperature is a difference between liquid refrigerant temperature as measured on the liquid line, and leaving condenser water temperature. Generally, the condenser approach for a water-cooled chiller will be in the range of 0 to 4 degrees. In case of any increase in this condenser approach, then it's a direct indication of fouling in a condenser.

How to check the condenser approach in water-cooled chiller:

Since the condenser approach is a quantity measured in terms of temperature any temperature measuring instrument can support checking this. RTD's can be installed in liquid refrigerant line and condenser leaving water line and condenser approach can be determined accurately. Else a simple IR temperature gun can be used in determining the temperatures. In recent days, all bigger range chillers are getting this measurement as an inbuilt feature. These systems will continuously monitor the condenser approach and even notifies the user when the condenser approach is exceeding the specified limits.

Methods to address high condenser approach problem:

• The most common practice in addressing this condenser approach is going for a chemical descaling or deep cleaning of condenser tubes. This requires shutting down of chiller system for a few hours and a lot of manual work involved in dismantling the tube system, cleaning, and rearranging. And since this involves a lot of chemicals it's not even considered as a sustainable way of cleaning.

• Another technology that emerged in recent days is the "Online Automatic Tube Cleaning System" (ATCS). This system will continuously monitor the condenser approach and will clean the tubes whenever the limits are exceeding with the help of small soft synthetic balls. These balls will be stored in a small reservoir placed in the loop with condenser circuit and whenever it's required these balls will rush into the condenser tube loop, cleans the tube, and avoids fouling. 

Effectiveness of ATCS:

Periodic mechanical cleaning will have a reduction in the condenser approach but will again gradually increase as time passes before the next cleaning cycle. In ATCS this problem has been addressed effectively as the approach is maintained constant throughout the operating period. As per one technology supplier for an 800 TR water-cooled chiller without ATCS, the average condenser approach was around 7 Degree F and with ATCS the same was observed as 2 Degree F.

This ATCS system doesn't require any shutdown and can operate while the system is online. Since there are no chemicals involved in cleansing the condenser tubes, this can also be considered as sustainable also.

The below video by one of the reputed technology suppliers clearly explains the concept of ATCS with fantastic animation.

Condenser Approach in Air-Cooled Condensers = Difference between Saturated Refrigerant Condensing Temperature to that of  Ambient Air Temperature. An Air-cooled Condenser would typically have a full load approach value somewhere between 10-15°C

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