The recently constructed Civic Center relied on a 37-ton DX, vav air conditioning system, which serviced several large meeting rooms, a kitchen, and a small amphitheater.
The system was designed to handle the area under a heavy load of people, lighting, and kitchen use. Many times only one room was in use out of the entire area (usually a small group of people having a meeting). The large condensing unit was able, through unloading, to reduce the system capacity to approximately 12 tons of cooling.
A conventional mechanical hot gas bypass was installed at the air handler DX coil, and an AHU controller regulated the discharge air to the vav boxes at 55°F. A variable-speed drive controlled the air handler motor speed based on static pressure created by demand of the vav boxes.
The problem occurred on mild days with minimal load. “The condenser unit would start and within about one minute, the air handler discharge temperature would drop well below the desired 55° temperature, causing the condenser unit to cycle off on low limit.
“No matter what we tried with control strategies, the room was either too warm or too cold under minimal load conditions,” says Rick Casentini, director of technical services, Independence, Ohio.
Due to the rapid cycling of the condensing unit, proper dehumidification was impossible. It was discovered that the root of the problem was the mechanical bypass valve, which was unable to respond in time to the actual load conditions — not passing enough hot gas to overcome the low pressure and temperatures at the air handler. So Casentini asked Jeffrey Caldwell, J.J. Caldwell Mechanical Inc., Breckenridge, CO, who was a technical consultant and application engineer under contract with the City of Independence at the time, to find a solution.
What to Do
Researching various options, including some very creative control strategies and variable-speed controlling of the compressor in the condensing unit, didn’t result in any practical solutions.“We were of the opinion that a proportional digitally controlled valve, which could react immediately to changing pressure and temperature conditions at the evaporator, was needed,” says Casentini.
Unfortunately, Casentini and Caldwell could not locate a manufacturer that produced such a valve. Most manufacturers they contacted indicated that they experienced similar problems with low-load cycling on DX systems; however they stated that the customers just had to live with the problem. Casentini and his staff refused to accept that answer.
At the suggestion of a colleague, Casentini contacted Sporlan Valve, which agreed to discuss the possibility of developing such a valve to be tested in Independence. The first test model valve worked perfectly, says Casentini.
A side loop control program was developed in the existing AHU controller to control the air handler discharge temperature between 54° and 56°. The newly developed valve controlled to within 4¼10 of a degree. After one minor mechanical fault was discovered and corrected, the valve worked perfectly.
So What Are They?
Electronically controlled valves are most often thought of as being electric expansion valves (EEVs). However, advancements in the technology have allowed the EEV to be applied in many other places in the modern refrigeration and air conditioning system.Typical EEVs were designed to open and close against high-side to low-side pressure drops. In an R-404A system at 100° condensing temperature and -20° evaporator temperature, this is about 220 psi. Given the correct motor power for the port size, this pressure differential is easily overcome, says Brian Dolin, director, Mechatronics Products Group, Sporlan Valve Co., Washington, MO.
“Liquid temperatures are usually in the range of 105° to 40° in expansion valve applications and don’t represent much of a challenge on the materials of construction of the valve or motor. A more demanding application is use as a discharge gas bypass valve,” or DBV.
The pressure drops experienced by DBVs are similar to those seen by TXVs or EEVs, but the temperatures are very different. When mounted close to the compressor, DBVs may have to regulate 230ü gas. Motor materials, synthetic seat and seals, and wire insulation must all be engineered to withstand this environment.
So why would anyone even think of using an electronically controlled valve in this manner? In a word, control, says Dolin.
“Mechanical DBVs are simply pressure regulators. They function by balancing the force of a spring against downstream pressure exerted on a diaphragm. Since the spring must compress and the diaphragm, must deflect to allow the port to open, all mechanical DBVs have hysteresis and gradient.”
(Hysteresis is the friction force of bending of the spring and diaphragm and usually causes the valve to close “easier” than it opens. This creates a gradient, meaning that the valve cannot control to an exact pressure under all flow conditions.)
Typical mechanical DBVs allow a 6° temperature change in the evaporator across their operating range. Precise temperature control with mechanical DBVs is therefore difficult or impossible, says Dolin.
“Electronically controlled DBVs or EDBVs have no gradient. They are electrically driven to a position, precisely and repeatably. This makes direct temperature control with EDBVs not only possible but also easy.”
Better Control a Benefit
Mechanical DBVs are not used as temperature controls but rather as capacity controls. As load on the system drops, evaporator pressure drops which, in turn, strokes the DBV open. The hot gas is used to falsely load the evaporator and compressor to keep it on-line.EDBVs, on the other hand, can be controlled with a temperature sensor or a pressure transducer. By mounting the temperature sensor in the air off the evaporator or in the outlet stream of chilled water, the EDBV will react with the proper amount of hot gas to both load the compressor and control the temperature directly.
“Our experience with process cooling manufacturers has shown that control on the order of plus or minus 0.1° is easily achieved. Some of our customers manufacture chillers for use in the medical, plastics, or instrumentation fields, where reliable sources of precisely controlled water for cooling are critical. No other system they have tried has been as simple, inexpensive and reliable as the EDBV with controller,” says Dolin.
Simple to Select and Install
Selecting an EDBV is really not that difficult. Unlike some mech-anical DBVs, EDBVs may be used with any refrigerant, provided the materials are compatible.EDBVs are motor operated and are therefore not reliant on the pressure-temperature relationship of the refrigerant. The five factors used for selecting an EDBV are: refrigerant, desired outlet fluid temperature, compressor capacity at minimum allowable evaporator temperature, minimum evaporator load, and condensing temperature at minimum load.
Unlike mechanical valves, says Dolin, oversizing an EDBV is not generally harmful. “If oversized by as much as 100%, EDBVs can still throttle to maintain setpoint. If undersized, of course, they will not have the capacity to maintain setpoint under fully reduced load.”
In addition, EDBVs are designed as cartridge-type valves; that is, all moving parts are replaced as a unit, and the body remains inline during service. Field problems seen with EDBVs have usually been related to abuse and contaminants more than any other cause. The good news is that EDBVs and other electric valves are more tolerant of dirt and can be commanded to open to purge contaminants.
As for the City of Indepen-dence, its administrators are thrilled with the valves. Says Casentini, “These valves have worked excellently in our facilities.
“Equipment stress and compressor failures have been minimized by the application of this product. A side benefit has been reduced energy consumption of the retrofitted units due to the elimination of excessive starting and stopping.”
