Dissecting HVAC Systems, Schemes and Mind Maps of Mechanics

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Air Conditioning

Clinic

Introduction to

HVAC Systems

One of the Systems Series

Introduction to

HVAC Systems

One of the Systems Series

A publication of Trane, a Business

of American Standard Companies

TRG-TRC018-EN iii

• period one Dissecting HVAC Systems Contents - Airside Loop - Chilled-Water Loop - Refrigeration Loop - Heat-Rejection Loop - Controls Loop
  • Chilled-Water Systems period two Direct-Expansion (DX) Versus
    • Direct-Expansion (DX) Systems
    • Chilled-Water Systems
    • Factors Affecting the Decision
• period three Common HVAC System Types - Single-Zone Systems - Multiple-Zone Systems
  • of the HVAC System period four Factors That Affect Selection
    • Preference of Building Owner
    • Available Construction Budget
    • Size and Shape of Building
    • Function of Building
    • Architectural Limitations
    • Life-Cycle Cost
    • Ease of Operation and Maintenance
    • Time Available for Construction
• period five Review
  • Quiz
  • Answers
  • Glossary

iv TRG-TRC018-EN

notes

Dissecting HVAC Systems

Some HVAC systems address these comfort requirements better than others.

In addition, there are other factors that affect comfort but are not directly related to the HVAC system. Examples include adequate lighting, and proper furniture and work surfaces.

The purpose of this period is to provide a method for understanding the components of different types of HVAC systems. The premise of this method is that any HVAC system can be dissected into basic subsystems. These subsystems will be referred to as “loops.” There are five primary loops that can describe virtually any type of HVAC system.

„ Airside loop (yellow) „ Chilled-water loop (blue) „ Refrigeration loop (green) „ Heat-rejection loop (red) „ Controls loop (purple)

Before we continue, keep in mind these three observations. First, while these five loops can be used to describe virtually any HVAC system, not every system uses all five loops.

Second, the temperatures used in this period are representative of conditions found in a typical HVAC system, but will differ from application to application.

And third, while another loop could be added for heating and humidifying the space in some systems, this clinic focuses primarily on comfort cooling, not heating.

The Five System Loops

AirsideAirside

Chilled waterChilled water

RefrigerationRefrigeration

Heat rejection Heat rejection

Controls Controls

Figure 4

Dissecting HVAC Systems

notes

Airside Loop

The first loop is the airside loop, and the first component of this loop is the conditioned space. The first two comfort requirements mentioned were dry- bulb temperature and humidity. In order to maintain the dry-bulb temperature in the conditioned space, heat (referred to as sensible heat) must be added or removed at the same rate as it leaves or enters the space. In order to maintain the humidity level in the space, moisture (sometimes referred to as latent heat) must be added or removed at the same rate as it leaves or enters the space.

Most HVAC systems used today deliver conditioned (heated, cooled, humidified, or dehumidified) air to the conditioned space to add or remove sensible heat and moisture. This conditioned air is called supply air. The air that carries the heat and moisture out of the space is called return air.

Imagine the conditioned supply air as a sponge. In the cooling mode, as it enters a space, this “sponge” (supply air) absorbs sensible heat and moisture. The amount of sensible heat and moisture absorbed depends on the temperature and humidity, as well as the quantity, of the supply air. Assuming a fixed quantity of air, if the supply air is colder, it can remove more sensible heat from the space. If the supply air is drier, it can remove more moisture from the space.

Airside Loop

supply airsupply air

sensiblesensible heatheat moisturemoisture (latent heat)(latent heat)

return airreturn air

conditionedconditioned spacespace

Figure 5

Dissecting HVAC Systems

notes

The next component of the airside loop is a supply fan that delivers the supply air (SA) to the space. In the example in Figure 7, air is supplied to the conditioned space to maintain a desired temperature of 75ºF (23.9ºC) in the space.

This same supply fan is often used to also draw the return air out of the space. Alternatively, some systems use a second fan, called a return fan, to draw air from the space and move it back to the equipment that contains the supply fan.

Another one of the comfort requirements is to provide an adequate amount of fresh, outdoor air to the space. In this example, the required amount of outdoor air (OA) for ventilation is brought into the building and mixed with the recirculated portion of the return air (RA). The remaining return air, that which has been replaced by outdoor air, is exhausted as exhaust air (EA) from the building, often by an exhaust (or relief) fan. In this example, outdoor air at 95ºF (35ºC) dry bulb mixes with recirculated return air at 75ºF (23.9ºC) dry bulb. This mixture contains 25 percent outdoor air and 75 percent recirculated return air, so the resulting temperature of the mixed air (MA) is 80ºF (26.7ºC) dry bulb.

Another comfort requirement is to ensure that the air in the conditioned space is clean. Bringing in an adequate amount of fresh outdoor air, and exhausting some of the air from the space, can help meet this requirement. However, the air must also be filtered. In a typical HVAC system, the mixed air passes through a filter to remove many of the airborne contaminants.

95°F 95°F (35°C)(35°C)

Supply Fan and Filter

supplysupply fanfan

conditionedconditioned spacespace

RA RA

OAOA EAEA

75°F 75°F (23.9°C)(23.9°C)

80°F80°F(26.7°C)(26.7°C)

SASA

filterfilter MAMA

Figure 7

notes

Dissecting HVAC Systems

As mentioned earlier, during the cooling mode, the supply air must be cold enough to absorb excess sensible heat from the space and dry enough to absorb excess moisture (latent heat). A heat exchanger, commonly known as a cooling coil, is often used to cool and dehumidify the supply air before it is delivered to the space.

In this example (Figure 8), the cooling coil cools and dehumidifies the entering mixed air from 80ºF (26.7ºC) dry bulb to a supply-air temperature of 55ºF (12.8ºC) dry bulb.

A typical cooling coil includes rows of tubes passing through sheets of formed fins. A cold fluid, either water or liquid refrigerant, enters one header at the end of the coil and then flows through the tubes, cooling both the tubes and the fins.

Cooling Coil

header header

tubestubes

finsfins

95°F95°F (35°C)(35°C)

RARA

OAOA EAEA

75°F75°F (23.9°C)(23.9°C)

80°F80°F(26.7°C)(26.7°C)

SASA

MA

coolingcooling coilcoil

55°F55°F (12.8°C)(12.8°C)

Figure 8

notes

Dissecting HVAC Systems

Finally, the supply air is delivered to the conditioned space. The temperature and humidity of the supply air, however, are only applicable for one point in time. The cooling requirement (load) in the conditioned space varies throughout the day and throughout the year. The airside loop responds to changing cooling loads in the conditioned space by varying either the temperature or the quantity of air delivered to the space.

A constant-volume system provides a constant quantity of supply air and varies the supply-air temperature in response to the changing cooling load in the space. A thermostat compares the dry-bulb temperature in the conditioned space to a setpoint. It then modulates cooling capacity until the space temperature matches the setpoint.

part-load operation

Constant-Volume System

65°F65°F (18.3°C)(18.3°C)

SASA

constant supply-air quantity variable supply-air temperature

95°F95°F (35°C)(35°C)

RARA

OAOA EAEA

75°F75°F (23.9°C)(23.9°C)

80°F80°F(26.7°C)(26.7°C) filterfilter^ MAMA

Figure 10

Dissecting HVAC Systems

notes

A variable-air-volume (VAV) system varies the quantity of constant- temperature supply air in response to the changing cooling load in the space.

In this system, a VAV terminal unit is added to the airside loop. Each conditioned space, or group of similar spaces (called a zone), has a separate VAV terminal unit that varies the quantity of supply air delivered to that space or zone. The VAV terminal unit contains an airflow modulation device, typically a rotating-blade damper.

A thermostat compares the dry-bulb temperature in the conditioned space to a setpoint. It then modulates the quantity of supply air delivered to the space by changing the position of the airflow modulation device in the VAV terminal unit. The capacity of the supply fan is modulated to deliver only the quantity of supply air needed, and cooling capacity is modulated to maintain a constant supply-air temperature.

95°F 95°F (35°C)(35°C)

RA RA

OAOA EAEA

75°F 75°F (23.9°C)(23.9°C)

80°F80°F(26.7°C)(26.7°C)

MAMA

part-load operation

Variable-Air-Volume (VAV) System

VAVVAV terminalterminal unitunit

SASA 55°F 55°F (12.8°C)(12.8°C)

variable supply-air quantity constant supply-air temperature

airflowairflow modulationmodulation devicedevice

VAVVAV terminal unitterminal unit Figure 11

Dissecting HVAC Systems

notes

mixed air passes through the filters, the supply fan, and the cooling coil before being discharged from the air handler.

Unlike the example fan-coil unit that was installed in the conditioned space, the central air handler needs a method for delivering the supply air to the conditioned space(s).

A supply-air distribution system, typically constructed of sheet-metal ducts, fittings, and diffusers, is used to direct the supply air from the central air handler to one or more conditioned spaces. The example airside loop in Figure 14 includes a central air handler and ductwork to deliver supply air to multiple VAV terminal units.

From each VAV terminal unit, the supply air travels through a section of flexible duct to remotely located diffusers. Diffusers are used to distribute the supply air effectively to the conditioned space. Proper air diffusion is an important comfort consideration, especially in VAV systems, to avoid dumping cold supply air on the occupants of the space.

Supply-Air Distribution System

VAVVAV terminalterminal

diffuserdiffuser

flexibleflexible sheetsheet--metal supply ductmetal supply duct ductduct

centralcentral air handlerair handler

outdooroutdoor--air inletair inlet

return-return-airair inletinlet

Figure 14

notes

Dissecting HVAC Systems

In Figure 15, air returns from the conditioned space to the central air handler through an open ceiling plenum. The plenum is the space between the ceiling and the roof, or floor, above.

Alternatively, a separate return-air duct system could be used to direct the return air back to the air handler.

Chilled-Water Loop

In the airside loop, a cooling coil is used to cool and dehumidify the supply air. As mentioned, the cold fluid flowing through the tubes of the coil may be either water or liquid refrigerant. Systems that use water flowing through the cooling coil also contain a chilled-water loop.

Heat energy flows from a higher-temperature substance to a lower-temperature substance. Therefore, in order for heat to be transferred from the air, the fluid

Ceiling Plenum Return

roofroof plenumplenum^ return airreturn air

ceilingceiling

diffuserdiffuser supply airsupply air

Figure 15

Chilled-Water Loop

42°F42°F(5.6°C)(5.6°C)

57°F57°F (13.9°C)(13.9°C)

55°F55°F (12.8°C)(12.8°C)

coolingcooling coilcoil

(26.7°C)(26.7°C)80°F80°F

Figure 16

notes

Dissecting HVAC Systems

refrigerant inside the tubes, and chilled water leaves from the opposite end of the shell.

The third component of the chilled-water loop is a pump that moves water around the loop. This pump needs to have enough power to move the water through the piping, the evaporator, the tubes of the coil, and any other accessories installed in the chilled-water loop.

Similar to the airside loop, the chilled-water loop responds to changing cooling loads by varying either the temperature or the quantity of water delivered to the cooling coil. The most common method, however, is to vary the quantity of water flowing through the cooling coil by using a control valve. As the cooling load decreases, the modulating control valve reduces the rate of chilled-water flow through the coil, decreasing its cooling capacity.

42°F42°F (5.6°C)(5.6°C)

57°F57°F(13.9°C)(13.9°C)

(12.8°C)(12.8°C)55°F55°F

coolingcooling coilcoil

80°F80°F (26.7°C)(26.7°C) evaporator evaporator

Pump and Control Valve

control control valvevalve

pumppump

Figure 19

Dissecting HVAC Systems

notes

At part-load conditions, a two-way control valve reduces the rate of chilled- water flow through the coil. A three-way control valve also reduces the rate of flow through the coil, but it bypasses the excess water to mix downstream with the water that flows through the coil.

With a three-way valve, the quantity of water flowing through the system (water flowing through the coil plus water bypassing the coil) is constant at all loads. With a two-way valve, the water flowing through the system varies, which allows the pump to reduce its capacity and save energy at part load.

Notice that the control valve is located at the outlet, or downstream, of the cooling coil. This location ensures that the tubes inside the coil are always full of water. A valve located at the inlet, or upstream, of the coil may modulate to the point where the water just “trickles” through the tubes, not filling the entire tube diameter. The result is unpredictable heat transfer and less-stable control.

Two-Way Versus Three-Way Valves

coil with two- coil with two-wayway control valvecontrol valve

coil with threecoil with three--wayway control valvecontrol valve

Figure 20