AE 573: HVAC Systems, Equipment, and Energy Efficiency
Estimated study time: 10 minutes
Table of contents
Sources and References
Primary texts — McQuiston, Parker, and Spitler, Heating, Ventilating, and Air Conditioning: Analysis and Design, 6th ed. (Wiley). ASHRAE, ASHRAE Handbook — HVAC Systems and Equipment and HVAC Applications, current editions.
Supplementary texts — Kreider, Curtiss, and Rabl, Heating and Cooling of Buildings, 3rd ed. (CRC Press). Mitchell and Braun, Principles of Heating, Ventilation, and Air Conditioning in Buildings (Wiley). Stoecker, Design of Thermal Systems, 3rd ed. (McGraw-Hill).
Online resources — ASHRAE Standard 90.1 and 62.1. AHRI 210/240 (Unitary) and AHRI 550/590 (Chillers). U.S. DOE Energy Efficiency and Renewable Energy Guide to Low-Energy Building Design. MIT OpenCourseWare 4.42J Fundamentals of Energy in Buildings. International Institute of Refrigeration technical notes on heat pumps.
Chapter 1: Foundations of HVAC System Design
1.1 Purpose and Performance Metrics
An HVAC system conditions space to satisfy four simultaneous constraints: thermal comfort per ASHRAE 55, acceptable indoor air quality per ASHRAE 62.1, acoustic performance, and regulated energy use per ASHRAE 90.1. Performance is quantified by seasonal efficiencies: COP for instantaneous heating and cooling delivery, SEER and HSPF for annualized unitary equipment, and IPLV for chillers under a weighted load profile.
1.2 Load-to-System Map
Design loads computed via the Radiant Time Series or Heat Balance methods give peak sensible and latent requirements. Systems are then chosen to match zoning, simultaneity of heating and cooling, part-load shape, and available energy sources. The canonical families are all-air, air-water, all-water, and refrigerant-based (direct-expansion and variable-refrigerant-flow).
Chapter 2: Air-Based Distribution
2.1 Single-Duct Constant Volume
Constant-volume reheat serves spaces through a single cold deck with terminal reheat to meet part-load heating. The sensible heat ratio line on the psychrometric chart from supply state S to room state R yields
\[ \dot{V}_s = \frac{\dot{Q}_{\text{sens}}}{\rho c_p (T_R - T_S)} . \]Although simple and quiet, constant reheat pays an energy penalty proportional to the mixed-cold-deck overcooling.
2.2 Variable-Air-Volume
VAV systems modulate zone airflow to meet sensible load, reheating only when flow reaches a minimum ventilation setpoint. Supply fan power scales roughly with the cube of volumetric flow, so VAV with a variable-speed drive yields the dominant energy advantage:
\[ W_{\text{fan}} = \frac{\dot{V}\,\Delta p}{\eta_{\text{fan}}\eta_{\text{motor}}\eta_{\text{drive}}} \propto \dot{V}^3 . \]Control sequences (ASHRAE Guideline 36) couple supply-air temperature reset, static pressure reset, and zone trim-and-respond to minimize reheat and fan energy.
2.3 Dedicated Outdoor Air Systems
DOAS decouples ventilation from zone conditioning by handling 100% outdoor air to neutral or slightly cool supply, while parallel systems (radiant panels, VRF, chilled beams) absorb zone sensible load. Latent control concentrates at the DOAS coil, allowing dry, efficient operation of the sensible system.
Chapter 3: Hydronic Systems
3.1 Chilled- and Hot-Water Loops
Water is 3500× denser than air per unit volume and transports heat with far less pump energy than fans. A chilled-water coil load is
\[ \dot{Q} = \dot{m}_w c_{p,w}(T_{\text{rtn}} - T_{\text{sup}}) = \rho\, \dot{V}_w c_{p,w} \Delta T . \]Primary–secondary and variable-primary configurations trade mechanical simplicity against pumping energy. Low delta-T syndrome — chillers producing cold water that returns nearly as cold — destroys plant efficiency; high-\( \Delta T \) design (7–9 K) is a central discipline.
3.2 Pump and Piping Hydraulics
Pump power for a given flow is \( W_p = \rho g Q H / \eta_p \). Head \( H \) is the sum of static lift and friction, the latter from Darcy–Weisbach
\[ h_f = f \frac{L}{D} \frac{V^2}{2g} , \]with \( f \) from the Colebrook equation or the Haaland explicit form. Balancing valves, two-way vs three-way control valves, and differential-pressure reset all determine whether a variable-speed pump realizes cubic savings or merely throttles against a near-constant head.
Chapter 4: Vapour-Compression Refrigeration and Heat Pumps
4.1 The Ideal Reverse Cycle
A Carnot heat pump operating between reservoirs \( T_L, T_H \) has
\[ \mathrm{COP}_{H,\text{Carnot}} = \frac{T_H}{T_H - T_L}, \qquad \mathrm{COP}_{C,\text{Carnot}} = \frac{T_L}{T_H - T_L} . \]Real vapour-compression cycles fall short because of finite-temperature heat exchange, isenthalpic expansion, and compressor irreversibilities. Second-law efficiency \( \eta_{II} = \mathrm{COP}/\mathrm{COP}_{\text{Carnot}} \) is typically 0.3–0.5 for modern equipment.
4.2 Cycle Analysis
On a pressure–enthalpy diagram the four processes are evaporation (1→2), compression (2→3), condensation (3→4), and expansion (4→1). Performance follows directly:
\[ \mathrm{COP}_C = \frac{h_2 - h_1}{h_3 - h_2} . \]Refrigerant selection balances thermodynamic performance, global warming potential, flammability class (A1 through A3), and pressure class. Transition from HFCs toward low-GWP blends (R-454B, R-32) and natural refrigerants (CO₂, propane, ammonia) is mandated by the Kigali amendment and regional f-gas regulations.
4.3 Air-Source and Geothermal Heat Pumps
Air-source units degrade at low ambient due to reduced \( T_L \) and frost-defrost cycling. Cold-climate variable-capacity inverter units maintain COP ≳ 2 to −20 °C by oversized evaporators, enhanced vapour injection, and optimized defrost triggers. Ground-source systems exploit stable ground temperature \( \approx \) annual mean dry-bulb. Vertical borehole design uses the Eskilson g-function:
\[ T_f(t) = T_g - \frac{q}{2\pi k_s} g(t/t_s, r_b/H) - q R_b , \]where \( R_b \) is borehole resistance and \( g \) encodes long-term thermal interaction among boreholes.
Chapter 5: Heat and Energy Recovery
5.1 Sensible and Enthalpy Wheels
Rotary regenerators transfer sensible and (with desiccant coating) latent energy between supply and exhaust streams. Effectiveness \( \varepsilon = (T_{s,\text{out}} - T_{s,\text{in}})/(T_{e,\text{in}} - T_{s,\text{in}}) \) typically reaches 0.75–0.85. Cross-contamination and purge sectors must be sized to preserve IAQ.
5.2 Run-Around Coils and Plate Heat Exchangers
Run-around coils separate exhaust from supply physically, at the cost of lower effectiveness (0.45–0.65) and added pumping power. The effectiveness–NTU relation
\[ \varepsilon = 1 - \exp\!\left[\frac{(NTU)^{0.22}}{C_r}\left(\exp(-C_r (NTU)^{0.78})-1\right)\right] \]applies for cross-flow with both fluids unmixed.
5.3 Economizer Cycles
When outdoor enthalpy is below return enthalpy, modulating dampers to 100% outdoor air eliminates mechanical cooling. The integrated-economizer control points (dry-bulb, fixed-enthalpy, differential-enthalpy) are chosen based on climate and sensor reliability.
Chapter 6: Part-Load Performance and Controls
6.1 Part-Load Performance Curves
Equipment efficiency is rarely constant. AHRI 550/590 defines IPLV as a weighted integral at 100%, 75%, 50%, and 25% load, with outdoor conditions varied appropriately. The BIN method and hourly simulation extend this to site conditions and actual load shapes.
6.2 Control Sequences and Commissioning
Sequences of operation govern how equipment responds to sensors. PID tuning balances rise time against overshoot; loop instability at low load is a leading cause of simultaneous heating and cooling. Functional performance testing during commissioning verifies that sequences behave as designed under realistic disturbances. Ongoing commissioning (monitoring-based commissioning) uses data analytics to detect drift, failed dampers, stuck valves, and schedule override.
6.3 Integration with the Building
Efficient HVAC cannot compensate for a poor envelope. Sequencing design — reduce loads, simplify distribution, select efficient equipment, control tightly, commission rigorously, monitor continuously — is the disciplinary contract of the energy-conscious HVAC engineer, and it underpins every standard discussed in this course.
