Guiding Your Way to Correct Chiller Piping
All piping should be installed by a licensed plumbing contractor in compliance with local codes. Never use galvanized piping if glycol is used in the chilled water system. Galvanic action, the chemical reaction between glycol and galvanized pipe, can harm the cooling system, the glycol and the chiller. Piping material may be copper, plastic or carbon or stainless steel, depending on installation and local code requirements.
The design engineer or piping contractor has the responsibility to ensure that the piping and the available dynamic head of the pump installed inside the chiller are sized correctly. The chiller's chilled-water pipe connections are not necessarily the appropriate size for the system's piping. As a general guide, the chiller pipe connections should be the minimum size required for the installation. Drastic reductions in pipe sizing reduce chilled water flow and may cause a low flow alarm or freezing damage in the evaporator.
Installations with low water flow and corresponding high water temperature rise always should have a full-ported bypass installed between the chiller inlet and outlet connections with a manually adjustable gate valve in the bypass line. Correctly adjusted, this blended return water maintains an adequate flow through the chiller at an acceptable return temperature to the chiller.
Always install a suction-pressure gauge, which is a compound gauge reading both positive and negative pressure, in the return piping close to the chiller, if it is not already installed in the unit. This is essential for monitoring the system's pressure and pump performance at startup and during operation.
It is good piping practice, especially on systems with short piping runs or low system pressure loss, to install a gate valve in the chiller's discharge line for throttling purposes. This allows the operator to maintain optimum pump performance by adding resistance to the system, thereby allowing the pump to perform at its optimum point on the curve. Never throttle the water flow on the return line to the chiller because it will cause pump cavitation and overheating.
Automatic Water MakeupIf the chilled-water cooling system is expected to lose water during normal operations such as mold changeover or hose-changing, an automatic water makeup system should be installed or ordered from the factory as an option.
The automatic makeup system must include a water-pressure regulator and pressure gauge. Be aware that if there is a tank installed inside the chiller, it will have a specified maximum design pressure rating. Do not discharge city water, which can be as high as 60 to 80 psig, directly into the chiller without first confirming the pressure rating of the chiller's tank. Instant pressurization could rupture the tank. Closed tanks in chillers always should have a relief valve for this purpose.
Do not use an automatic water makeup system if glycol is installed for antifreeze protection, because the glycol will become diluted in time and the freeze protection will be lost. Some critical cooling applications such as medical equipment manufacturing may require installation of emergency city water and drain solenoid valves for protection in the event of chiller failure. In such applications, the chiller must be isolated via automatic or manual valves from city water pressure to avoid damage from overpressurization.
After any activation of the emergency city water system, the glycol concentration must be checked carefully using a hand-held spectrometer. A relatively inexpensive instrument, it allows the operator to check glycol concentration in a simple 10-sec procedure at any time. It is far more accurate than the simple float-type device used by automotive radiator shops. If indicated, add glycol to maintain the correct antifreeze concentration.
Expansion TankSome closed-loop chillers are supplied with an expansion tank fitted inside. Its purpose is to absorb hydraulic expansion of the water, which can damage the chiller, piping or equipment to be cooled. It also provides positive pressure on the suction side of the circulating pump. For systems with extensive piping, or large chilled-water volumes, it is best to install a closed-diaphragm expansion tank in the return chilled-water line close to the chiller, if it is not supplied inside the chiller.
The tank should be sized using the appropriate calculations for the volume of water in the system, and the maximum expected temperature fluctuation of the water in the piping system, under all possible conditions. The expansion tank should be connected to the side of a vertical pipe or the bottom of the horizontal return piping so that entrained air bubbles circulating with the chilled water are not trapped in the expansion tank.
Expansion tanks typically are factory pre-charged with compressed air to approximately 12 to 20 psig. After installation, filling and venting of the entire system, expansion tank pressure should be set to provide a pressure of 5 to 10 psig on the return side of the chiller with the chiller in operation. Air pressure can be added or removed from the diaphragm tank via a Schraeder valve.
The vertical piping immediately connected to and from the chiller will impose a static, or standing, head pressure that can be read on the chiller's pump gauges when the system is not operating. For example, if the supply or return piping from the chiller rises 15' before running above a ceiling, the gauge static pressure will show approximately 6 to 7 psig. (To calculate psig, multiply the distance in feet by 0.424.) This is simply the weight of the water in the vertical piping at that location and does not indicate an overall system pressure.
System VentingThe single most common problem in chiller piping installations is lack of chilled-water flow caused by inadequate system venting. The result is repeated flow alarms when the flow switch that is installed in the chiller opens the control circuit, stops the chiller and sets the alarm. These chillers are closed-circuit systems and normally are not open to atmosphere, which means that air remains in all local high points when the system initially is filled with water.
A local high point is any point in the piping that can be described as an inverted "U." More clearly defined, if the piping rises vertically anywhere and at any elevation in the system, travels horizontally for any distance, and then drops again vertically, the inverted "U" section is a permanent air lock and must be vented. Venting is required at all local high points and on both the supply and return pipes.
Vents can be either manual or automatic. Automatic vents always should be installed with an isolation valve for future service access, repair or replacement. Automatic vents are particularly susceptible to drawing air into the return chilled-water piping close to the chiller if the suction side of the chilled water pump is allowed to fall into a vacuum.
Positive PressurizationIt is good practice to maintain a slightly positive pressure in a closed-loop cooling system to:
Positive system pressure can be imposed by carefully and slowly introducing city water pressure via a hose connection anywhere in the system. After the system is completely filled and vented, note the gauge pressure at the supply and return of the chiller. If there is no significant vertical piping connected directly to the chiller, the gauges should be around zero. If a pre-pressurized expansion tank is installed in the system, the initial system pressure should be approximately that of the expansion tank. Start the chiller and observe the operating pressure on both the return and discharge water-pressure gauges. The return line to the chiller while operating should be approximately 5 to 10 psig, confirmed by the return line pressure gauge. The discharge gauge on the chiller should be at the optimum point of the pump curve, depending on the piping and system pressure losses of the installation.
The pump's return, or suction, pressure can be raised or lowered by carefully using city water pressure via a hose connection, by adjusting the diaphragm pressure of the expansion tank, by adjusting the discharge pressure via a throttling valve, or by using a combination of these methods.
Measured when the system is not operating, the expansion tank normally should not be pressurized to more than 20 psig. Generally, these expansion tanks have a factory rating of 75 to 100 psig, but the rating can be greater for high pressure applications. The final-discharge pump pressure should never be allowed to exceed the maximum pump-rating pressure.
A word of caution: Supercharging the pump with city water, which could push up water-discharge pressure to higher than the pump's nominal rating, can damage the pump seal, chiller, piping system or process equipment being cooled.
If glycol is required for antifreeze protection, always use an industrial inhibited ethylene glycol or food-grade propylene glycol. Never use automotive antifreeze. Exercise great caution in the selection of a glycol supplier, and confirm the freeze protection after installation using a spectrometer. Damage caused by freezing can be very expensive.
Glycol can be premixed with water to the correct concentration and then pumped into the system, or pumped in separately from the water if system capacity is calculated accurately. Alternatively, the glycol or water can be filled from the system high point through a funnel or hose connection.
The most common method for filling the system is to pump the water/glycol into the chiller's fill or drain connection using a hose or portable pump with all system vents open. There is usually a manual or automatic vent in the chiller tank for initial filling and venting purposes, in addition to the high point piping vents installed by the contractor.
The system should be filled slowly and carefully, allowing all air to escape. The system can only be filled as fast as the air can escape. Be patient and do not overpressurize the system. After initial operation, check that all air is vented. After operating the system for several hours, all vents should be checked again to ensure that all entrained air has escaped.
If the piping is installed in accordance with these guidelines, the closed-loop cooling system should operate satisfactorily. Most importantly, the chilled water flow should be at the optimum curve of the circulating pump without problems. PCE