The Digital Fix
Maintain a consistent chilled water temperature during varying load conditions.
May 1, 2012 by Dave Demma
Like it or not, we live in a digital world. Today’s vehicles are controlled by computers, our children are constantly plugged into something electronic and at work we cannot function without computers. Even the HVAC/R industry is becoming digitized. Electric expansion valves with electronic controllers are becoming more prevalent. Nearly all supermarket refrigeration systems are controlled and monitored by a controller.
So, it should come as no surprise that a recent mechanical problem was resolved with a digital solution. The issue was with a chiller providing the comfort cooling for a facility that housed historic archives.
The chiller was equipped with two scroll compressors. They were not the digital scrolls, which have the ability to operate unloaded. These scrolls had no unloading capabilities. The compressor staging allowed for the chiller to operate at 50 per cent capacity (one compressor operating) or 100 per cent capacity (both compressors operating). Due to the nature of the archival material, the humidity parameters for the building were critical, and to maintain the desired humidity the chilled water temperature requirement was 42F.
While there were two compressors on the chiller, the chiller barrel was configured as a single circuit with a single mechanical TEV providing the refrigerant mass flow.
To maintain a constant 42F chilled water temperature with the normal load variations between day and night conditions, and the extreme near 100F ambient temperature experienced in the summer contrasted with the more mild temperatures in the winter with one step of chiller capacity reduction was an impossible task.
The contractor maintaining the equipment decided to install a discharge bypass valve as a means to maintain a consistent saturated suction temperature. The discharge bypass valve was piped such that the gas would be introduced into the system in the suction line. This meant a de-superheating TEV was required to prevent the compressor from overheating. The reasoning was that if the saturated suction temperature could be kept from falling below a pre-determined pressure point, it would prevent the chilled water temperature from falling below the desired set-point of 42F.
The logic was sound. This is a typical application for a discharge bypass valve; preventing the saturated suction pressure from falling below a predetermined level. However, the limitation in attempting to maintain a consistent chilled water temperature during varying load conditions, varying water flow rates and varying ambient temperatures, by simply keeping the saturated suction temperature from falling below 42F is one tall order. It did not produce the desired results.
After a bit of brainstorming, it was decided that an electronic discharge bypass valve (see Figure 1) would be used in place of the mechanical discharge bypass valve. However, this application had one very significant difference. The electronic discharge bypass valve, along with its controller, would not be set up to maintain the saturated suction pressure from falling below a pre-determined point. Rather, the controller would be set up to monitor and maintain the chilled water temperature at 42F. The controller sensor was secured to the chilled water outlet on the chiller barrel. The hot gas was injected into the chiller barrel inlet, through a nozzle adaptor (a modified brass tee), which allows for the gas to be injected such that it does not immediately interact with the liquid/vapour mixture leaving the TEV.
With this configuration, the controller is able to sense the actual system condition that is desired to be controlled; not another system condition that has some influence on the condition that is desired to be controlled.
The result should not have been too surprising, since electric valves/electronic controllers are very precise and very repeatable, despite varying system conditions. After the installation was complete, and the controller set correctly, the chilled water temperature was held constant, with a +/- 0.3F variance.
Yes, we live in a world that is becoming more and more digital every day. And why not? The absolute precision, reliability, and repeatability provided with digital control cannot be approached with mechanical control. <>
Dave Demma holds a degree in refrigeration engineering and worked as a journeyman refrigeration technician before moving into the manufacturing sector where he regularly trains contractor and engineering groups. Contact him at firstname.lastname@example.org.