Beyond the three main design objectives of safety, control and reliability, there are several other factors that will influence the system design. Noise and vibration must be addressed. Access to replaceable components must be available. The materials of construction in each component must be compatible with the entire range of conditions that device could face. These details and many other design considerations will not be discussed here.
In this chapter, we will discuss typical components used to promote safety, control, and reliability in four types of systems: liquid, steam, refrigerant, and air. At the end of the chapter, you will also be introduced to the basic components found in the pneumatic control system and the electrical power supply system. Your goal in this chapter is to become familiar with the symbol used to represent each component. You should also be able to state the function and typical location of each item.
Overpressure problems are avoided by the installation of a pressure relief valve. This device will automatically open at a preset pressure. When the pressure has dropped sufficiently, the valve closes again. On domestic hot water systems, a pressure and temperature relief valve is used. These respond to both high pressure and high temperature conditions.
The main hazard due to temperature is personnel burns. Any surface at a temperature above 140°F can cause second degree burns. These dangers can be prevented through proper pipe and equipment insulation. To prevent scalding when a pressure relief valve goes off, the discharge from each device must be piped to a drain or other safe disposal site. Any surface colder than about 60°F should be insulated to prevent condensation which can drip and damage items below. This moisture source can also allow the growth of mold and mildew, which can create an IAQ problem.
Control of liquid flow is achieved with various types of valves. A check valve is used to prevent fluid from going backwards: it allows flow in only one direction. If two parallel pumps are installed in a system but only one of them is running, a check valve at the pump discharge can prevent water from going back through the second pump.
A gate valve is used to isolate equipment and piping loops. A gate valve on both the suction and discharge sides of each pump would allow the pump to be removed for maintenance while the system continues to operate on the parallel pump. A globe valve is used to control the liquid flow rate. Most kitchen faucets are this type. Because the pressure loss through an open globe valve is much higher than through a gate valve, globes should not be used for isolation applications. Conversely, gate valves are a poor choice for flow control situations, since they allow most of the flow to occur when they are only about 10% open. a triple-duty valve is often specified for the pump discharge. This single valve serves the function of a check valve, isolation valve, and flow control valve. One trait that all three of these valves share is a linear action to operate (the valve opens as the valve stem rises).
There are two other valves that require a quarter turn of rotation between fully closed and fully open. Ball valves can be used for isolation, flow control or both. In small sizes, ball valves are usually the cheapest option. In large diameter piping systems, butterfly valves are frequently specified. These valves require the least opening torque.
Three way valves are also commonly used in hydronic heating systems, although the name is really a misnomer. While these valves do have three ports, the fluid enters one port and exits either the second or third port, depending on the valve setting. Three-way valves are also available with two entry ports and a common discharge port.
All of these valves can be operated manually, or fitted with an actuator and controlled automatically. When on/off control is required, a solenoid valve is specified. When the flow must be allowed to vary from closed to full capacity, a modulating valve is selected. Many types of systems require the installation of a bypass valve around a piece of equipment to control the flow rate through the equipment.
Reliability of liquid systems focuses on the control of three conditions: air, freezing and corrosion. Air does not belong in closed liquid systems, and must be removed with air vents. These are generally located near the top of the system (because air tends to rise), or immediately after heat transfer equipment (since air is less soluble in hot water than in cold water). On the other hand, in some cases air must be brought in through a vacuum breaker to get a system to function properly. Just like holding your finger over the end of a straw full of water, pipes can retain water until air is allowed to enter the top. In cold weather, this retained water could freeze and rupture the pipe.
Freezing of pipes, coils and other equipment must be prevented at all times. Most susceptible systems are simply drained in the fall, which means piping runs must be properly sloped. A second alternative is to use a non-freezing solution in the system. Three commonly used solutions are ethylene glycol (used in your car radiator, but highly toxic), propylene glycol (more expensive but non-toxic) and brine (salt is cheap but corrosive). If none of those are possible, then pipes must be well insulated and electric or steam tracer lines installed to provide enough heat to prevent freeze-up.
Several types of corrosion can attack liquid piping systems. The worst case is galvanic corrosion, which is caused by having two dissimilar metals (say copper and steel), in contact, in an electrolytic solution (like water). The copper (or other cathode material) draws electrons from the steel (or other anode material), allowing the steel to dissolve quickly. Designing a battery like this into your system can cause the system to leak in a matter of months. If you must have two metals present, separate them with a dielectric coupling to avoid premature failure. There is at least one case where galvanic corrosion is encouraged. In water heaters, a sacrificial anode made of magnesium is used to prevent attack on the steel tank itself.
Another common form of corrosion, fouling, results from insoluble chemicals that plate out on heat exchange surfaces and can decrease performance and service life. Scale forms on the inside of pipes due to oxidation, or the formation of metal oxides and rust. Getting the air out of a system is a good first step to minimizing oxidation. When flakes of scale and rust become entrained in the water flow, they can damage pumps, and plug up control valves. A strainer is usually installed before these components to collect this debris. When antifreeze solutions become overheated, the glycol breaks down into an acid that quickly attacks the pipe. Chemical additives are also used to alleviate these problems. Oxygen scavengers, for example, will seek out and bind up oxidants. Buffers can also be added to ensure that the fluid does not turn acidic. Other additives can prevent fouling by dissolving the scale.
Control of steam systems is generally accomplished by using the boiler outlet pressure or temperature to adjust the fuel valve. All of the valves discussed above are also used in steam systems. In addition, a pressure control valve is sometimes used to maintain desired conditions. A steam trap is used to allow the condensate (steam condensed back into water in the coil or radiator) to return to the boiler but not the steam. Proper sloping of steam and condensate lines is important to the successful operation of these systems.
Water chemistry is extremely important to the longevity of a boiler. The concentration and condition of additive must be carefully monitored, and the chemical feed rates adjusted regularly by the operator. Even with good water quality control, blowdown, or removal of water from the boiler or cooling tower, is required to prevent a buildup of solids in the unit. Makeup water must be added to replace the blowdown and any leaks from the system.
The expansion valve between the condenser and the evaporator is the primary control element in these systems. Bypass valves are occasionally used for flow control. In a heat pump, the reversing valve is used to change from the heating mode to the cooling mode.
Several components are used to increase refrigeration system reliability. A suction accumulator on the suction of the compressor ensures that no liquid droplets get ingested into the compressor. A filter/drier ensures that clean, water-free refrigerant is running through the system. The oil used to lubricate the compressor must be compatible with the refrigerant and it condition must be monitored regularly to prevent degradation. Proper system charging with the right quantity of the right refrigerant is the responsibility of the refrigeration mechanic.
The greatest safety problem with air systems is during a fire emergency. Smoke and heat are easily conducted through the ductwork. Fire and smoke in a duct can be sensed with high temperature alarms and smoke detectors respectively. Automatic fire dampers must be installed in ducts penetrating fire walls, and all penetrations must be adequately sealed. The controls of the mechanical system can be integrated with the fire control system.
Control of both airflow and air pressure through ductwork systems is required for proper operation. This is accomplished by either changing the fan speed with a variable speed drive (VSD), or by using some type of damper. A vortex damper installed on the inlet of the fan is generally the most efficient type of damper to use. Both parallel blade and opposed blade dampers are used to control airflow through coils and into conditioned spaces. Balancing dampers found in residential forced air systems and manually-adjusted commercial air systems are usually the butterfly type. With the exception of balancing dampers, almost all dampers are automatically controlled.
The location, type, and size of ceiling diffuser selected for an air-conditioning application are critical to providing uniform and stable conditions throughout the controlled zone. The air flow at the point of use is generally controlled by a VAV-box, which is also called an air valve.
The electrical power supply to each motor must also include several components for safe and reliable operation. A local disconnect switch must be located near each motor to protect maintenance workers from being electrocuted. Remote or automatic start capability is provided by a magnetic starter or contactor (a switch in the power lines that is operated by the control circuitry). Fuses or circuit breakers to protect the wires from overload are included in the local disconnect or starter. Since motors draw significant electrical amperage on startup, they are usually also protected from startup overload by heaters (fuses that allow brief high currents before tripping).
An Uninterruptible Power Supply or UPS must often be provided to provide power to the computer controls throughout the facility. The Emergency Generator system is also often located within the mechanical room. This system must be supplied with fuel and combustion air, and the vent products must be ducted outside the building (often through a muffler to reduce noise). An automatic electrical crossover switchgear panel must be located nearby. In buildings where cogeneration is used, the electrical generating and distribution systems as well as the waste heat boiler become the responsiblities of the physical plant staff.
Finally, when the building is protected by a fire sprinkler system, the various components associated with that system. These might include the fire pump to maintain water pressure when the system is in use, and a small jockey pump to overcome pressure losses when the system is not in use. There may be one or more water source tanks throughout the facility. Many building codes also require a test location to measure the system flow rate under various conditions. There will also be a fire safety computer that monitors smoke detectors, temperature detectors and manual pull boxes throughout the building. It activates appropriate fire alarms and may have a direct line into the local fire station. This unit might also be interlocked with the HVAC control panel to automatically adjust damper positions based on the location of the problem. This same signal might also start a fan to pressurized the stairwells, and activate a smoke removal system.