Air-Handling Unit Components
You should recognize Figure 6.2, which was originally introduced in Chapter 2, Figure 2.12. It shows the basic air-handler unit with the economizer cycle. Some new details have been added in this diagram. In the following section, we will go through each of the components in the unit, we will discuss what each component does, and, in general terms, how each component can be controlled. This unit is typically referred to as the single-zone air handler.
The overall functions of the air-handler are to draw in outside air and return air, mix them, condition the mixed air, blow the conditioned air into the space, and exhaust any excess air to outside.
Air Inlet and Mixing Section
The inlet louver and screen restrict entry into the system. The inlet louver is designed to minimize the entry of rain and snow. A very simple design for the inlet louver is shown in the diagram. Maintaining slow air-speed through the
louver avoids drawing rain into the system. More sophisticated, and more costly designs allow higher inlet-velocities without bringing in the rain. The screen is usually a robust galvanized-iron mesh, which restricts entry of animals, birds, insects, leaves, etc.
Once the outside air has been drawn in, it is mixed with return air. In Figure 6.2, a parallel blade damper is shown for both the outside air damper and the relief air damper.
These dampers direct the air streams toward each other, causing turbulence and mixing. Mixing the air streams is extremely important in very cold climates, since the outside air could freeze coils that contain water as the heating medium. A special mixing section is installed in some systems where there is very little space for the mixing to naturally occur.
It is also possible to install opposed blade dampers:
These do a better job of accurately controlling the flow, but a rather poorer job of promoting mixing.
Some air will be exhausted directly to the outside from washrooms and other specific sources, like kitchens. The remainder will be drawn back through the return air duct by the return air fan and either used as return air, or exhausted to outside through the relief air damper. This exhausted air is called the relief air. The relief air plus the washroom exhaust and other specific exhaust air will approximately equal the outside air that is brought in. Thus, as the incoming outside air increases, so does the relief air. It is common, therefore, to link the outside-air damper, the return-air damper and the relief- air dampers and use a single device, called an actuator, to move the dampers in unison. When the system is “off,” the outside-air and relief-air dampers are fully closed, and the return-air damper is fully open. The system can be started and all the air will recirculate through the return damper. As the damper actuator drives the three dampers, the outside-air and relief-air dampers open in unison as the return-air damper closes.
Mixed Temperature Sensor
Generally, the control system needs to know the temperature of the mixed air for temperature control. A mixed-temperature sensor can be strung across the air stream to obtain an average temperature. If mixing is poor, then the average temperature will be incorrect. To maximize mixing before the temperature is measured, the mixed temperature sensor is usually installed downstream of the filter.
When the plant starts up, the return air flows through the return damper and over the mixed temperature sensor. Because there is no outside air in the flow, the mixed-air temperature is equal to the return-air temperature. The dampers open, and outside air is brought into the system, upstream of the mixed-air sensor. If the outside temperature is higher than the return temperature, as the proportion of outside air is increased, the mixed-air temperature will rise. Conversely, if it is cold outside, as the proportion of outside air is increased, the mixed-air temperature will drop. In this situation, it is common to set the control system to provide a mixed-air temperature somewhere between 55 and 60°F. The control system can simply adjust the position of the dampers to maintain the set mixed temperature.
For example, consider a system with a required mixed temperature of 55°F and return temperature of 73°F. When the outside temperature is 55°F, 100% outside air will provide the required 55°F. When the outside air temperature is below 55°F, the required mixed temperature of 55°F can be achieved by mixing outside air and return air. As the outside temperature drops, the percentage required to maintain 55°F will decrease. If the return temperature is 73°F, at 37°F there will be 50% outside air, and at 1°F, 20% outside air.
If the building’s ventilation requirements are for a minimum of 20% outside air, then any outside temperature below 1°F will cause the mixed temperature to drop below 55°F. In this situation, the mixed air will be cooler than 55°F and will have to be heated to maintain 55°F. The mixed-air temperature-sensor will register a temperature below 55°F. The heating coil will then turn “on” to provide enough heat to raise the supply-air temperature (as measured by the supply-temperature sensor) to 55°F.
Now let us consider what happens when the outside-air temperature rises above 55°F. Up to 73°F, the temperature of the outside air will be lower than the return air, so it would seem best to use 100% outside air until the outside temperature reaches 73°F. In practice, this is not always true, because the moisture content of the outside air will influence the decision. In a very damp climate, the changeover will be set much lower than 73°F, since the enthalpy of the moist, outside air will be much higher than the dryer return air, at 73°F. Above the pre-determined changeover temperature, the dampers revert to the minimum ventilation rate, 20% outside air in this example.
The last few paragraphs have discussed the how the system is controlled, called the control operation. These control operations can be summarized in the following point form, often called the control logic:
When system off, the outside air and relief air dampers fully closed, return air dampers fully open.
When system starts, if outside temperature above 70°F, adjust dampers to provide x cubic feet per minute (cfm) of outside air.
When system starts, if outside temperature below 70°F, modulate dampers to maintain 55°F mixed temperature with a minimum of x cfm of outside air.
The requirement for a minimum volume of outside air means that the controller must have a way of measuring the outside air volume. This can be achieved in a number of ways that are explained in the ASHRAE Course Fundamentals of Air System Design1.
The preceding text has talked about air volumes without getting into spe- cific numbers. Note that the weight (mass if you leave earth) of outside air entering the building must equal the weight of air that leaves the building. The volume of air that is entering and leaving will usually be different, since the volume increases with increasing temperature. For example:
82 lb/min, 1000 cfm of outside air, at 25°F, enters a building. It is heated, and leaves the building as 82 lb/min, 1100 cfm at 75°F (10% greater volume, same weight)
Filter
All packaged units include as least minimal filters. Often it is beneficial to specify better filters, as we discussed in Chapter 4.4.
Heating Coil
Some systems require very high proportions, or even 100% outside air. In most climates this will necessitate installing a heating coil to raise the mixed air temperature. The heat for the heating coil can be provided by electricity, gas, water or steam.
The electric coil is the simplest choice, but the cost of electricity often makes it an uneconomic one.
A gas-fired heater often has the advantage of lower fuel cost, but control can be an issue. Inexpensive gas heaters are “on-off” or “high-low-off” rather than fully modulating. As a result, the output temperature has step changes. If the unit runs continuously with the heat turning on and off, then the supply temperature will go up and down with the heater cycle and occupants may experience a draft.
Hot water coils are the most controllable, but there is a possibility that they will freeze in cold weather. If below-freezing temperatures are common, then it is wise to take precautions against coil freezing. Many designers will, therefore, include a low-temperature alarm and arrange the controls to keep the coil warm or hot, when the unit is off during cold weather.
This is one of the times when the designer needs to take precautions against the consequences of the failure of a component. If, for example, the damper linkage fails, the unit may be “off,” with the outside dampers partially open to the freezing weather. The consequence, a frozen coil, is serious since it will take time to get it repaired or replaced.
Cooling Coil
Cooling is usually achieved with a coil cooled by cold water, or a refrigerant. The cold water is normally between 42°F and 48°F. There are numerous refrigerants that can be used, and we will discus the refrigerant cycle and how it works in the next section. Whether using chilled water or a refrigerant, the coil will normally be cooler than the dew point of the air and thus condensation will occur on the coil. This condensation will run down the coil fins to drain away.
With refrigeration coils in packaged systems, there is limited choice in the dehumidification capacity of the coil.
Humidifier
A humidifier is a device for adding moisture to the air. The humidifier can either inject a water-spray or steam into the air.
The water-spray consists of very fine droplets, which evaporate into the air. The supply of water must be from a potable source, fit for human consump- tion. If impurities have not been removed by reverse osmosis or some other method, the solids will form a very fine dust as the water droplets evaporate. This dust may, or may not, be acceptable.
The alternative is to inject steam into the air stream. Again, the steam must be potable.
The humidifier will normally be controlled by a humidistat, which is mounted in the space or in the return airflow from the space. Excessive operation of the humidifier could cause condensation on the duct surface and result in water dripping out of the duct. To avoid this possibility, a high humidity sensor is often installed in the duct, just downstream from the unit. In addition, one might not want the humidifier to run when the cooling coil is in operation.
The unit control logic will then be:
Humidifier off when unit off
Humidifier off when cooling in operation
Humidifier controlled by space humidistat when unit in operation
Humidifier to shut down until manually reset if high limit humidity sensor
operates
Fan
The fan provides the energy to drive the air through the system. There are two basic types of fan, the centrifugal, and the axial.
Within the centrifugal fan, air enters a cylindrical set of rotating blades and is centrifuged, thrust radially outwards, into a scroll casing. This fan is a very popular choice due to its ability to generate substantial pressure without excessive noise.
The other type of fan is the axial fan, where the air passes through a rotating set of blades, like an aircraft propeller, which pushes the air along. This is a simpler, straight-through design that works really well in situations that require high volumes at a low pressure-drop. When this type of fan is made for really low pressure-drops, wide pressed-sheet-metal blades are used and it is called a propeller fan.
Return Fan
A return fan is usually included on larger systems, unless there is some other exhaust system to control building pressure. If there is no return fan, the building will have a pressure that is a bit above ambient (outside). In a hot, humid climate, this is beneficial since it minimizes the infiltration of outside air into the building, where it could cause condensation and mildew. In cold climates, the excess pressure above ambient can cause leakage of moist air into the wall, where it freezes and causes serious damage.
Having briefly reviewed the unit components, we are going to take time to consider the refrigeration cycle and its operation.