🔑 Key Thermodynamic Properties You Should Know

🧪 2️⃣ Specific Humidity (γ) — The Moisture Pie 🍰

Alright, let’s continue our journey into the moist world of air! 😄 You already know that Humidity Ratio (W) tells us how much water vapor exists per kilogram of dry air.

But what if you’re not just interested in dry air… what if you want to know:

“Out of the whole chunk of air — both dry and moist — how much of it is actually water vapor?”

That’s where Specific Humidity (γ) comes in. It gives us the water vapor as a slice of the total air mass pie. 🥧

📖 What Is Specific Humidity?

👉 Specific Humidity (γ) is defined as the mass of water vapor divided by the total mass of moist air (i.e., dry air + water vapor).

So instead of comparing water vapor to only the dry portion (like we do with WWW), we now compare it to the entire air mixture.

🔍 The Formula (Straight from ASHRAE Ch.1)

Where:

• M_w = Mass of water vapor
• M_da​ = Mass of dry air

✨ But here’s the cool part — if you already know the humidity ratio WWW, you can calculate γ in one step:

🎉 This makes your psychrometric life a whole lot easier — no need to measure vapor mass and dry air mass separately!

💡 Let’s Break It Down with a Story

Picture you’ve got a balloon filled with air 🎈. It contains:

• 1 kg of dry air
• 0.02 kg of water vapor

That means:

• Total air = 1 kg + 0.02 kg = 1.02 kg
• Specific humidity =0.02/1.02=0.0196

So, around 1.96% of that air’s mass is water vapor — that’s γ.

Isn’t that a nice, bite-sized fact about your air? 😄

🧠 Why Is Specific Humidity Useful?

While engineers love the humidity ratio in HVAC system design, specific humidity is super helpful in:

✅ Atmospheric science
✅ Weather forecasting
✅ Modeling airflow in climate zones
✅ Psychrometric calculations when dealing with mass transfer

Because it’s normalized to the total air mass, it’s easy to compare across different conditions.

🔍 Small Numbers, Big Impacts

You might be wondering: “Why are these numbers so small?”

That’s because even humid air is mostly dry air — over 98% nitrogen and oxygen, with just a little sprinkle of H₂O molecules. But that tiny bit of moisture drives cooling loads, comfort levels, and even weather systems. 🌦️

So yes, specific humidity might look small, but it packs a punch! 💥

🔄 Relationship Between γ and W

As a reminder:

These formulas are like siblings. If you know one, you can instantly find the other — which is super handy when moving between atmospheric and engineering calculations.

📊 Real-World Example (Using ASHRAE Data)

Let’s use the standard atmosphere data at 30°C and 100% relative humidity. According to ASHRAE Table 2:

• Humidity Ratio WWW ≈ 0.0273 kgw/kgda
  Now plug it into our equation:

So, 2.66% of the total mass of the air is water vapor when the air is fully saturated at 30°C.

It doesn’t sound like much, but it’s enough to fog your windows, sweat your brows, and make your AC work overtime 😅.

📘 Pro Tip from ASHRAE (Ch.1)

Specific humidity is often used interchangeably with absolute humidity in non-technical texts — but they are not the same!

Make sure to use the correct one for the job! ✅

📦 Summary — Why γ Is So Great

📌 So, What Is Relative Humidity?

Relative Humidity (φ) is the ratio of:

💧How much water vapor is actually in the air
✨ TO
💦How much water vapor the air could hold if it were saturated

In short, it tells us how close the air is to being full of moisture.

🧠 Think of It Like a Sponge

Let’s say air is like a sponge 🧽. It can soak up a certain amount of water vapor, depending on the temperature.

• If it’s holding half as much moisture as it can → φ = 50%
• If it’s fully soaked (can’t hold more) → φ = 100%
• If it’s super dry → φ = 10%

φ = 100% = saturation = dew point reached = condensation starts = fog, sweat, or dew happens 💧🌫️

✏️ The ASHRAE-Approved Formula

According to the ASHRAE Handbook, the formula for relative humidity (when assuming perfect gas behavior) is:

🌟 φ is usually expressed as a percentage: just multiply the result by 100!

This is a simplified but accurate equation when dealing with general HVAC and psychrometric problems. ASHRAE also provides more complex versions that include enhancement factors when high accuracy is needed (e.g., meteorology labs), but this one works great in most real-world HVAC applications.

🧪 Where Do These Pressures Come From?

• p_w (actual vapor pressure): How much water vapor is actually present in the air.
• p_ws (saturation pressure): How much water vapor the air could hold if it were completely saturated at that temperature.

📌 ASHRAE provides accurate saturation pressure values in Table 3 (Chapter 1) or you can calculate them using the Hyland-Wexler equation (Equation 6).

🧊 Example Time!

Let’s say you’re in a room at 25°C and someone measured:

• Actual water vapor pressure: pw=2.5 kPap_w = 2.5 \text{ kPa}pw​=2.5 kPa
• Saturation vapor pressure at 25°C: pws=3.17 kPap_{ws} = 3.17 \text{ kPa}pws​=3.17 kPa (from ASHRAE Table 3)

Now plug into the formula:

✅ That means the air is holding about 79% of the maximum moisture it can hold at 25°C.

So it’s not fully saturated, but it’s definitely getting there… and it may feel sticky 🥵.

🧠 How Is φ Different From Humidity Ratio (W)?

Great question!

• W = mass of water vapor per mass of dry air
• φ = how “full” the air is, relative to its capacity at that temperature

In fact, there’s a beautiful connection between φ and W. ASHRAE defines the saturation humidity ratio W​ at temperature t, and then:

But remember: φ is a relative property — and that’s what your body perceives most. Not the absolute amount of water in the air, but how close it is to saturation.

🧊 φ = 100% Means…?

Whenever relative humidity hits 100%, any more water vapor will condense out.

This leads to:

• 💧 Dew forming on surfaces
• 🌫️ Fog in the atmosphere
• 💦 Sweat not evaporating → discomfort
• 🧊 Water collecting in AC coils (condensate)

☀️ φ Depends on Temperature — Even If Moisture Stays the Same!

Here’s a mind-blowing fact:

If the amount of moisture in the air stays the same, but the air warms up, φ goes down.
If the air cools down, φ goes up.

Because warm air can hold more water vapor than cold air! That’s why:

• It feels dry in winter — cold air holds less water vapor, so φ drops inside heated buildings.
• It feels humid after a summer rain — the temperature and moisture both stay high.

🧠 Bonus: Complex ASHRAE Definition (When You Need High Accuracy)

For precision systems, ASHRAE uses:

Where:

• f(p,t)f(p, t)f(p,t) = enhancement factor (accounts for non-ideal behavior)
• e(t)e(t)e(t) = vapor pressure at temp ttt
• tdt_dtd​ = dew point temp

But unless you’re doing lab-level climate modeling or metrology, stick with the simplified formula we showed earlier!

🌟 Real-Life Applications of φ

• 📦 HVAC System Design: Proper φ control ensures indoor comfort.
• 🏠 Building Science: High φ can lead to condensation → mold risk! 😬
• 🧪 Process Cooling: Some manufacturing processes need φ below 30%.
• 🧊 Dehumidification: AC systems often target φ between 40–60%.

What You Should Remember

Calculating Relative Humidity (φ) from Dew Point Temperature (td)

This is the easiest and most direct method if you know:

• The dry-bulb temperature (t) (a.k.a. room temperature)
• The dew point temperature (td) (when the air becomes saturated)

✅ Step-by-step:

🔹 Step 1: Get the vapor pressure at dew point p_w=e(t_d)

This is the actual partial pressure of water vapor in the air.

📘 From ASHRAE Table 3 (Ch. 1) or using Hyland-Wexler Equation (6):

🔹 Step 2: Get saturation vapor pressure at dry-bulb p_ws=e(t)

This is the maximum vapor pressure air can hold at its current temperature.

📘 From the same source:

Step 3: Apply the simplified formula

Boom! 💥 You just calculated relative humidity from dew point!

Calculating φ from Wet-Bulb Temperature (t*)

This is a bit more involved, but super useful — especially when you’re working with just a psychrometer (those two thermometers, one wet and one dry)!

You need:

• Dry-bulb temperature (t)
• Wet-bulb temperature (t*)
• Barometric pressure (p) (usually 101.325 kPa at sea level)

✅ Step-by-step:

Let’s say:

Step 2: Use the wet-bulb humidity formula:

The relative humidity φ can be approximated from the following empirical psychrometric relation (based on conservation of energy and mass during evaporation):

Where:

• A = psychrometric constant ≈ 0.00066 (for natural ventilation, metric units)
• p = barometric pressure in kPa
• t = dry-bulb temp (°C)
• t∗ = wet-bulb temp (°C)
• p_ws(t∗) = sat. pressure at wet-bulb
• p_ws(t) = sat. pressure at dry-bulb

Step 3: Plug in values

→ from ASHRAE Table 3: p_ws(30∘C)≈4.243

✅ That’s your relative humidity based on wet-bulb temperature!

🎓 Summary: Two Pathways to φ

🔥 4️⃣ Enthalpy (h) — The Total Heat Story of Moist Air

You’ve heard people say, “This air feels warm and heavy,” or “The AC’s on, but it’s still muggy.” 😅

Behind that feeling is not just temperature… it’s heat content, or in fancy thermodynamic terms:
👉 Enthalpy, symbolized as h.

If you’re designing an air conditioning system, calculating heat loads, or sizing a humidifier or dehumidifier — you can’t escape enthalpy. It’s your best friend 💼🔥

Let’s unwrap it. 👇

📚 What Is Enthalpy in Plain English?

Enthalpy is like a heat energy scorecard for moist air.
It tells you:

❓ How much total energy (heat) does this air contain?

But we’re not just talking about dry air — we’re talking about moist air, which is a mix of:

• Dry air (mostly nitrogen and oxygen)
• Water vapor (H₂O in gas form)

Both parts hold heat. And that’s what enthalpy measures:

The total heat in 1 kg of dry air + its associated water vapor.

🔥 Sensible Heat vs. Latent Heat

Here’s the trick: Enthalpy of moist air is made of two parts:

👉 Enthalpy combines both. That’s why it’s so useful in air conditioning: it lets you account for both temperature changes and moisture changes at once!

✅ So, enthalpy includes:

• The heat stored in dry air, plus
• The heat stored in the water vapor mixed with that air

✏️ ASHRAE’s Standard Formula for Enthalpy (Ch. 1)

h = h_da + W x h_g

Where:

✅ So, enthalpy includes:

• The heat stored in dry air, plus
• The heat stored in the water vapor mixed with that air

h_da ​≈ 1.006 x t

(Where t = dry-bulb temp in °C)

h_g​ ≈ 2501 + 1.86 x t

Why these numbers?

🧪 So the full working formula becomes:

h = 1.006t + W (2501+1.86 x t) [h=1.006t+W(2501+1.86t)]

This is the ASHRAE go-to equation for psychrometric calculations and design.

🎓 What Units Are We Talking About?

Enthalpy is usually expressed in:

• kJ per kg of dry air
  (Why dry air? Because W already accounts for water vapor, so we normalize to dry air mass)

So if:

t = 30^∘C W = 0.020 kgw/kgda

Plug it into the formula:

h = 1.006(30) + 0.020(2501+1.86⋅30) = 30.18 + 0.020(2501+55.8) = 30.18 + 0.020(2556.8) = 30.18 + 51.14 = 81.32kJ/kgda

🎯 So your air has 81.32 kJ of total heat in every kg of dry air. This includes both the warm air and the invisible steam (water vapor) mixed with it.

💡 Why Is Enthalpy So Useful in HVAC?

Because it tells us how much heat energy we need to add or remove from air!

For example:

• Want to cool air from 30°C to 20°C and remove moisture?
  ➡️ Use enthalpy to calculate how much total energy you need to take out.
• Want to heat air and humidify it at the same time?
  ➡️ Use enthalpy to calculate how much energy to add (both for warming and for evaporating water).

It’s a one-stop-shop for thermal load calculations in HVAC 🔁

📉 What Happens When Moisture Goes Up?

Let’s say you’re still at 30°C, but the air is more humid, say W = 0.025

Now enthalpy becomes:

h = 1.006(30) + 0.025(2501 + 1.86⋅30) = 30.18 + 0.025(2556.8) = 30.18 + 63.92 = 94.10kJ/kgda

👀 That’s a huge jump in heat content — just because we added a small amount of water vapor!

That’s the power of latent heat — and that’s why humidity control is such a big deal in HVAC.

What You Should Know About Enthalpy (h)

🧊 Can Enthalpy Be Negative?

YES! 😲 When air is very cold (like –20°C) and has very little moisture, the total enthalpy can dip below zero. ASHRAE Table 2 shows negative values for enthalpy at low temperatures and very dry conditions.

📊 ASHRAE Tables & Psychrometric Charts

ASHRAE Chapter 1 gives detailed Table 2 with enthalpy values for moist air at different temperatures and humidities. These values are also plotted directly on psychrometric charts as diagonal lines. 📈

🌡️ 5️⃣ Thermodynamic Wet-Bulb and Dew-Point Temperatures — The Hidden Truths in the Air

Ever wondered why sweating works great in dry weather but makes you feel sticky in the tropics? Or why your car gets covered in dew early in the morning? 😮

These everyday moments have names in the HVAC world:

• Dew-Point Temperature
• Thermodynamic Wet-Bulb Temperature

These two are superstars in psychrometrics — and once you understand them, you’ll unlock some serious air-handling power! 🔓💨

🧊 What Is Dew-Point Temperature (td)?

Let’s start with the dew-point temperature, often just called dew point.

💧 Dew-point temperature is the temperature at which moist air becomes saturated (100% relative humidity) and water starts to condense — forming fog, dew, or water droplets.

It’s basically the “sweat point” of the air.
Go below it? 💧 Hello condensation!
Stay above it? The water stays invisible in vapor form.

🧠 How Does ASHRAE Define Dew Point?

According to ASHRAE Ch.1:

It is the temperature t_d at which the humidity ratio W of a moist air sample equals the saturation humidity ratio W_s(p,td) at pressure p.

Mathematically:

This means if you know the humidity ratio and pressure, you can solve for tdt_dtd​ — the temperature where the air will start giving up its water vapor.

💡 Real-Life Example

Let’s say:

• Humidity ratio W=0.012
• Pressure p=101.325kPa (standard atmosphere)

Find the temperature where that same WWW equals the saturation humidity ratio. That’s your dew point.

And guess what?
🌫️ When your room temperature cools to this point, you’ll get:

• Fog on mirrors
• Dew on grass
• Condensate on ductwork (and possibly… mold 😨)

🧊 Dew-Point Equations from ASHRAE

ASHRAE provides this handy formula (Equation 36):

Once you solve for the vapor pressure p_w​, you can use Hyland-Wexler equations or ASHRAE Table 3 to find the corresponding t_d.

📌 Shortcut formulas are also available for direct dew point calculation (for everyday HVAC use), like:

If dew point is between 0°C and 93°C:

Where α=ln(p_w​)

But unless you’re writing psychrometric software, a chart or calculator will do just fine for finding dew point in real life. 👍

🌬️ What Is Thermodynamic Wet-Bulb Temperature (t*)?

Now, let’s talk about the wet-bulb temperature — but not just any wet bulb…

🎓 We’re talking about the thermodynamic wet-bulb temperature (denoted t^∗) — the temperature at which water evaporates into air adiabatically to bring the air to saturation at that same temperature and pressure.

Translation? Let me paint a picture:

🧊💨 Imagine This:

You have a stream of warm, dry air.
You introduce liquid water 💧 into that air and let it evaporate, without adding or removing any other heat.
The air absorbs moisture and cools itself down.

It eventually reaches saturation — and the temperature it reaches at this point is…

👉 The thermodynamic wet-bulb temperature.

✏️ Approximate Formula for Wet-Bulb (for perfect gas assumption):

Using simpler expressions, ASHRAE gives:

🧠 Looks scary? Yes.
But this is baked into every psychrometric software and wet-bulb calculator out there.

For a quick estimate:

• At typical room conditions (say 25°C, 50% RH), wet-bulb might be ~18°C
• It’s always between the dry-bulb and dew-point temperatures

🧮 ASHRAE’s Energy Balance for Wet-Bulb

Here’s the big formula from ASHRAE (Equation 31):

Where:

h = initial enthalpy of air

W = initial humidity ratio

W_s^∗​ = saturation humidity ratio at t^∗

h_w^∗​ = enthalpy of water added (kJ/kg)

h_s^∗​ = enthalpy of saturated air at t^∗

📌 This equation just means:
The total heat in the air + evaporated water = heat content of saturated air at t*

It’s the golden equation for adiabatic cooling and evaporative systems. 💧🌬️

🧊 6️⃣ Compression Refrigeration Cycles — How We Make Things Cool!

Ever wonder how your air conditioner chills your room in the middle of a heatwave? Or how a refrigerator keeps your food fresh while the kitchen’s roasting?

All of that happens thanks to one magical thermodynamic loop:

👉 The Vapor Compression Refrigeration Cycle.

Let’s explore this refrigeration rockstar — one that’s been quietly working inside fridges, freezers, ACs, chillers, heat pumps… and even ice cream machines 🍦.

🧠 What Is a Compression Refrigeration Cycle?

In simple words:

It’s a closed-loop system that picks up heat from a cool space (like your room or a fridge) and dumps it somewhere warmer (like outdoors).

This sounds backward, right? Moving heat from cold to hot?
That’s against nature! 🌡️

So… how do we cheat nature?

💡 We use mechanical energy to drive a cycle that lets us do it — enter the vapor compression cycle.

🧰 The 4 Key Components of the Cycle

Let’s break down the core players in every vapor compression cycle:

This forms a loop, with the refrigerant (the working fluid) circulating over and over. 🔁

🌀 Step-by-Step Walkthrough of the Cycle

Let’s say we’re using R-134a (a common refrigerant). Here’s what happens:

🧊 Let’s Take a Ride Around the Cycle

🔁 Point 1 → 2: The Compressor (The Power Booster)

🛠️ What’s happening here?
The refrigerant enters the compressor as a low-pressure, cold vapor (point 1). It’s not very useful yet, but…

💪 The compressor acts like a pump on steroids. It squeezes the refrigerant, making it:

• High-pressure
• High-temperature
• Still in vapor form

📌 You’ll see W₂ next to the compressor — that’s the mechanical work being done by the compressor (usually from electricity).

⚡ In short: This is where energy is added to the system. Without this step, nothing moves!

🔁 Point 2 → 3: The Condenser (The Heat Dumper)

🌬️ What’s happening here?
Now we’ve got hot, high-pressure vapor at point 2 entering the condenser.

The job of the condenser? 👉 Get rid of the heat the refrigerant is carrying!

It does this by transferring heat to the surrounding air or water — you’ll see Q₃ and T₀ (environmental temp) near this component in the diagram.

As it gives off heat, the refrigerant condenses into a high-pressure liquid.

🧊 Imagine steam cooling down and turning into water — same idea.

🔁 Point 3 → 4: The Expansion Valve (The Pressure Dropper)

🎢 What’s happening here?
This is the big pressure drop moment! The refrigerant, still a high-pressure liquid, passes through a tiny nozzle or valve called the expansion valve.

The result? It expands rapidly, and its pressure drops dramatically, creating a cold mixture of liquid and vapor.

🔥 This is like blowing compressed air from a can — it comes out freezing cold, right?

This cold mixture is now ready to absorb heat in the next step.

🔁 Point 4 → 1: The Evaporator (The Heat Soaker)

❄️ What’s happening here?
At this point, the refrigerant enters the evaporator at a low pressure and low temperature.

Here’s where the magic happens:
🌡️ It absorbs heat from the refrigerated space (or the room, in the case of ACs).

That’s represented by Q₁ and Tᵣ in the diagram:

• Q₁ = Heat absorbed by the refrigerant
• Tᵣ = Refrigerated space temperature

As it absorbs this heat, the cold refrigerant boils and evaporates, turning fully into vapor — ready to go back to the compressor and start the cycle again!

👏 Boom! Cooling achieved.

🛠️ 7️⃣ Multistage Vapor Compression Systems — Why One Compressor Isn’t Always Enough

Okay, so you’ve learned how a basic vapor compression cycle works — with one compressor, one condenser, one expansion device, and one evaporator.

Great! 👏 That cycle works beautifully for normal temperature ranges — like cooling your home or your soda 🥤.

But what if you want to:

• Cool a blast freezer to –40°C? ❄️
• Maintain ultra-low temps for vaccines or ice rinks? 🧪🏒
• Do it efficiently without frying your compressor?

👉 Then it’s time to call in backup: multistage systems 💪

🎯 Why Go Multistage?

When we try to go really cold (say, evaporating at –30°C or lower), a single compressor cycle runs into trouble:

1. Huge pressure differences → very high compression ratios 😵
2. Super high discharge temperatures → bad for compressor lifespan 🧨
3. Low efficiency (bad COP) → expensive electricity bills ⚡

👉 That’s why we split the work across two (or more) compression stages — to keep things cool (literally and figuratively 😎).

🧊 What Is a Multistage Vapor Compression System?

It’s a system that uses two (or more) compressors (or a two-stage compressor) to compress refrigerant in steps, instead of doing all the work in one go.

We also add something special between the stages…

🎁 An intercooler (also called a flash chamber or economizer).

Let’s unpack that.

➤ 1. Low-Stage Compressor (1 → 2)

• Takes in low-pressure vapor from the evaporator
• Compresses it to intermediate pressure
• Sends it to the intercooler

➤ 2. Flash Intercooler (2 → 3a & 3b)

• Think of this as a cooling pit stop 💧
• Some refrigerant from the condenser line is expanded here
• This flash-expanded refrigerant cools the vapor coming from stage 1
• At the same time, separates the vapor and liquid

Now you’ve got:

• Cold vapor going to high-stage compressor
• Cold liquid going to evaporator

Super cool, right? 😄

➤ 3. High-Stage Compressor (3a → 4)

• Takes in intermediate-pressure vapor
• Compresses it to condenser pressure
• Sends it to the condenser to reject heat

➤ 4. Condensation + Expansion

• Condenser cools and condenses the refrigerant (4 → 5)
• Part of it goes to flash intercooler (for cooling + liquid separation)
• The rest goes through an expansion valve into the evaporator (5 → 1)

And the loop begins again 🔄

📊 P-h Diagram of a Two-Stage System

You’ll see two compression lines instead of one, and a mid-cycle flash process.

This helps visualize:

• Energy added by compressors
• Heat rejected at the condenser
• Flash chamber as a “node” between stages

👷 Real-World Applications

Multistage refrigeration is used wherever deep chilling or ultra-low temps are needed, such as:

👷 Real-World Applications

Multistage refrigeration is used wherever deep chilling or ultra-low temps are needed, such as:

🧠 So, What Is Absorption Refrigeration?

It’s a refrigeration cycle where heat energy, instead of a mechanical compressor, drives the refrigerant through the cycle.

It uses a pair of chemicals:

• A refrigerant (usually water or ammonia)
• An absorbent (usually lithium bromide or water, depending on the refrigerant)

Together, they form a working fluid pair that allows the cycle to operate using heat instead of a compressor. Neat, huh? 😎

💡 Quick Comparison: Compression vs. Absorption

🛠️ The Key Components of a Simple Absorption Cycle

Let’s talk about the Ammonia-Water system (good for below-zero applications):

☑️ What About the Lithium Bromide–Water System?

Water becomes the refrigerant
Lithium bromide (LiBr) is the absorbent

⚠️ Key note:
This system only works above 0°C, because water can’t evaporate below freezing without freezing.

It’s used in:

• Large HVAC systems (chillers)
• Where waste heat or steam is available
• Quiet and vibration-free operation

🔄 Cycle Walkthrough (Ammonia-Water)

Let’s take it step-by-step:

1️⃣ Generator (Boiler)

Heat is added here 🔥.
This boils ammonia out of a water-ammonia solution.
→ The ammonia vapor heads to the condenser, leaving water-rich solution behind.

2️⃣ Condenser

The hot ammonia vapor releases heat to surroundings (like in compression cycles)
→ It condenses into liquid ammonia.

3️⃣ Expansion Valve

The liquid ammonia expands, lowering pressure and temperature.
Now it’s a cold mixture, ready to absorb heat!

4️⃣ Evaporator

This cold ammonia evaporates, absorbing heat from the environment — and voilà, cooling happens! 🧊

5️⃣ Absorber

The ammonia vapor flows into the absorber, where it’s absorbed by water (the absorbent).
→ This forms a strong solution (rich in ammonia) and releases heat (which must be rejected).

6️⃣ Pump

This strong solution is pumped back to the generator, and the loop starts again!

🔄 And that’s an absorption cycle — no compressor needed! Just heat and chemistry. 🧪🔥

📊 Efficiency — Let’s Talk COP

In vapor compression, we use COP (Coefficient of Performance) to measure how efficiently we move heat.

Absorption systems typically have lower COPs:

Why lower? Because we’re using heat, which is a lower-grade energy compared to electricity. But it’s often “free” heat from other processes — so we’ll take it! 🔥♻️

🧠 What ASHRAE Says (Chapter 2 Highlights)

• Absorption systems are great for energy recovery from waste heat.
• They’re commonly found in industrial processes, district cooling, trigeneration (electricity + heat + cooling), and off-grid applications.
• The pump consumes very little energy, so even with lower COP, overall efficiency can be reasonable.
• Using double-effect or triple-effect cycles (with multiple generators and absorbers) boosts performance when high-temperature heat sources are available.

🌍 Real-World Applications

🧪 Bonus Perks of Absorption Systems

✅ Fewer moving parts → low maintenance
✅ No compressor noise or vibration
✅ Works well with low-pressure refrigerants
✅ Can run on solar, biomass, natural gas, or waste heat
✅ Environmentally friendly — often no ozone-depleting refrigerants

What You Should Remember

• Absorption refrigeration uses heat instead of electricity
• Great for systems with waste heat or solar thermal
• Key working pairs:
  🔹 Ammonia–Water → for sub-zero cooling
  🔹 Water–Lithium Bromide → for chilled water (above 0°C)
• Lower COP, but can save a lot of energy costs in the right settings!

🧊 Absorption vs Compression — Quick Recap

🧠 First, What Is a Refrigerant Mixture?

It’s a combo of two or more pure fluids (like R-32 and R-125) that together act as a refrigerant.

Some blends behave like a single, unified fluid. Others don’t.

The way these blends behave during boiling and condensation gives us our two categories:

🔶 Zeotropic Refrigerants – A Bit More Complex 🤔

These blends behave like a team of individuals — each component boils/condenses at its own pace.

So what happens?

• Boiling occurs over a temperature range, not a single temp
• Same for condensation
• This creates what’s called “temperature glide”

Examples:

• R-407C (R-32 + R-125 + R-134a)
• R-404A
• R-410A (technically near-azeotropic, but still has a slight glide)

📌 Think of it like a band with different instruments tuning in at different times 🎻🎺🎸

🔷 Azeotropic Refrigerants – Like a Perfect Team 🤝

These blends behave like a pure substance.

• They boil and condense at constant temperatures
• No component separation during phase change
• No "temperature glide"

Examples:

• R-500 (R-12 + R-152a)
• R-502 (R-22 + R-115)
• Some newer low-GWP blends designed to act azeotropic

📌 Think of them like synchronized swimmers — every molecule changes phase together, perfectly in sync 🏊‍♀️

✅ Easy to design systems for
✅ No composition shift
✅ Consistent performance over time

🌡️ What’s Temperature Glide?

Glide is the difference between:

• The bubble point: where the first bubble of vapor appears (boiling starts)
• The dew point: where the last drop of liquid vaporizes (boiling ends)

Or vice versa during condensation.

🔺 In zeotropes, boiling and condensation don’t happen at a flat temp — they glide across a few degrees (sometimes 5–7°C or more!)

This has some implications 👇

⚙️ System Design Considerations: Why It Matters

🧪 Where Does This Matter?

Quick Recap

🚨 What Is Fractionation?

In zeotropic blends, if only one component leaks, the remaining mixture changes its composition.

Result?
💥 The refrigerant’s properties shift — your system won’t behave as designed anymore.

That’s why zeotropic systems should always be charged as a liquid, so the correct blend ratio is maintained.

🧊 Real-Life Analogy: Ice Cream vs. Fruit Salad

Let’s say refrigerants are desserts 🍨🍓

• Azeotropic = ice cream
  🟰 Uniform blend; scoop anywhere and it tastes the same
• Zeotropic = fruit salad
  🟰 Pick one spoonful, and you might get more banana than strawberry — depends on how it was mixed!

This is exactly what happens in systems using zeotropes. You have to manage the blend carefully to keep things balanced.

📖 ASHRAE’s Take (Ch. 2 Highlights)

• Zeotropic blends can improve system performance when the glide matches the heat transfer profile of the evaporator or condenser.
• But they require careful design, especially for evaporator and condenser surface area.
• Isothermal heat exchangers are less efficient with zeotropes unless optimized for glide.
• Azeotropes simplify performance analysis and are easier to model for design software.

🧠 Wait… What Is Exergy?

Exergy is the maximum possible useful work you can extract from a system as it comes into equilibrium with its surroundings.

It’s also called:

• Available energy
• Availability
• Useful work potential

If energy is the total fuel, exergy is the part you can actually use to do something useful — like turning a fan, moving a piston, or powering a cooling coil.

🧮 How Do We Calculate Exergy?

Let’s simplify this using air or refrigerants as our working fluids.

✏️ For closed systems (thermomechanical exergy):

Exergy In=Exergy Out+Exergy Destroyed

Where:

🧊 The dead state is when the system is in perfect thermal, mechanical, and chemical equilibrium with its surroundings — basically, when it’s useless for work.

📉 Exergy Destruction Examples

Let’s say you're analyzing a vapor compression system. Where does exergy get destroyed?

In fact, the expansion valve is usually the biggest villain — all that pressure drop, but no work recovery 😤

⚙️ Exergy Efficiency

To measure how well we’re using available energy, we define Exergy Efficiency:

✅ It tells you how well a system is converting available energy into useful results, not just whether the energy balances.

🧠 What ASHRAE Says (Chapter 2 Insights)

• Exergy is not conserved like energy — it’s degraded by irreversibilities
• Heat transfer across large temp differences causes exergy destruction
• Throttling devices (like expansion valves) are major exergy killers
• Second Law analysis is key for evaluating true system performance, not just energy balance

🧊 Summary — Exergy in a Nutshell

🔁 Energy vs. Exergy (The Key Difference)

So while energy can’t be destroyed (First Law), exergy can be lost or destroyed — especially when a system is inefficient 😓

💡 So What Is Second Law Analysis?

While the First Law of Thermodynamics is about energy balance (in = out),
the Second Law is about exergy balance — tracking how much of that energy can actually do work before being degraded.

Exergy Balance Equation:

Exergy In=Exergy Out+Exergy Destroyed\text{Exergy In} = \text{Exergy Out} + \text{Exergy Destroyed}Exergy In=Exergy Out+Exergy Destroyed

Where:

• Exergy destroyed comes from irreversibilities (like friction, mixing, heat transfer across temperature gradients)

✅ Second Law analysis helps you find where losses occur and how to improve efficiency.

🔥 Where Does This Show Up in HVAC?

Oh, everywhere! Let’s see:

🧠 Engineers use exergy analysis to optimize components, compare systems, and reduce environmental impact.

🌍 Why Does Exergy Matter (Beyond Equations)?

Because exergy = sustainability.

Every time we waste exergy, we use more resources, burn more fuel, and increase emissions — even if energy totals balance.

ASHRAE and other sustainability-driven engineers are pushing for Second Law-based design, because:

• It highlights true inefficiencies
• It enables energy recovery opportunities
• It helps compare systems more realistically

🧰 Real-Life Example: Air Conditioner

Let’s say your AC consumes 3 kW of electricity to remove 9 kW of heat.

• Energy efficiency (COP) = 9 / 3 = 3
• But if ambient temp is 30°C and you’re cooling to 5°C, the theoretical max COP is much higher!

That means exergy analysis would reveal:

• Where energy was wasted
• How much potential was lost
• Which component is underperforming