Fluid Mechanics, HVAC & Refrigeration

Stand a column of fluid up and every layer has to carry the weight of everything above it.

Q = A v — Q equals A v — is the most-used relation in the whole of applied fluids. Q is the volumetric flow rate in m³/s, A is the cross-sectional area the flow passes through, in m², and v is the average velocity across that section, in…

Moving fluid carries energy simply by moving, and there are two customary ways to quote it. Both describe the identical physical fact; they differ only in the currency.

Follow one parcel of fluid along its path. If nothing adds energy and nothing takes it away, the three ways it can hold energy must add to a constant.

Before you can calculate friction in a pipe you have to know which KIND of flow you are looking at, because the two kinds obey different arithmetic.

Submerge anything and the fluid above the top face presses down while the fluid below the bottom face presses up — and because pressure grows with depth, the upward press always wins.

Push water down a pipe and the wall takes a toll.

Darcy–Weisbach is useless until something hands you f. Three relations will, and picking the wrong one is not a small error — it is an order of magnitude.

Darcy–Weisbach is exact and wants a friction factor, a roughness and a viscosity.

An elbow is barely 150 mm of pipe, and it can cost more head than twenty metres of straight run. The loss is not friction along a wall; it is the swirl and separation left behind when a stream is made to change direction.

K factors describe a fitting's shape. A control valve gets a different treatment entirely, because valve makers sell capacity, and capacity is defined by a test, not by a theory.

Both of this lesson's meters run on the same idea: narrow the pipe, and the stream must speed up to get through. Speeding up costs pressure, by Bernoulli, so the pressure drop across the restriction reports the flow.

Two relations, and they only mean anything side by side: one says what a pipe can hold, the other says what the water will throw at it.

Every chemical dose, every flush, every glycol charge and every expansion-tank calculation starts from one question: how much water is in there?

A pump does not know what a building looks like. It knows one number: the total dynamic head it has to develop, in metres of the liquid it is moving.

Power is work per second, and the work a pump does is lifting weight.

The hydraulic power is what the water receives. The shaft power is what the motor has to hand the pump to make that happen, and it is always the larger of the two, because impellers leak, recirculate, rub and stir.

Here is the sentence the whole lesson hangs on: a centrifugal pump does not suck. It lowers the pressure at its eye, and something else — atmosphere, a static level, a pressurised vessel — pushes the liquid in.

Change a centrifugal pump's speed and everything about it moves — but not by the same amount.

A drive is not the only way to take a pump down a size.

A pump curve says what the machine can OFFER at each flow.

Where this bites in the plant room is the re-sheave. A belt-driven fan's speed is set by pulley diameters, and a balancer chasing design airflow will change one.

Now the honest headline, and it is the whole lesson: this number is not dimensionless and it is meaningless without its convention.

Moist air is a mixture — dry air with a little water vapour stirred through it — and describing its state takes more than one number.

Everything in psychrometrics is measured against one curve: how much water vapour air can hold before it starts giving it back.

Relative humidity is a ratio to a target that moves.

Cool air far enough and its vapour pressure meets the saturation curve. From that instant the air can hold no more, and every further degree of cooling puts water on the nearest cold surface.

Wet a wick, whirl it through the air, and the water evaporates.

A coil does not care about temperature; it cares about ENERGY, and the energy in moist air is its enthalpy. h = 1.006t + W(2501 + 1.86t) — read aloud h equals one-point-oh-oh-six t, plus W times two-five-oh-one plus one-point-eight-six t.…

Every air handler starts with a mixing box: outdoor air for ventilation, return air from the space, and one blend leaving for the coil.

Draw a cooling coil's process on the chart and it runs as a straight line from the entering condition down toward the saturation curve.

Every load an airstream carries is one of three things, and the trade has a one-line rule for each. American HVAC knows them as 1.08, 0.68 and 4.5.

A cooling coil does two jobs at once, and the balance between them decides whether a room ends up comfortable or merely cold and damp.

Two unit traps live in that one line, and both are quiet. Duct is drawn in millimetres and air is scheduled in litres per second — neither is what the formula wants.

Nobody measures duct velocity directly. A pitot tube faces the airstream and brings a thread of it to rest, and the pressure that appears is the air's kinetic energy made visible.

Ventilation gets specified two ways. Sometimes it is a flow — so many litres per second per person. Sometimes it is a rate — so many times an hour the whole room's air is replaced.

This is the site's home turf.

A closed loop is a sealed box full of water that you then heat. Three things follow, and each has its own relation.

A radiator's catalogue output is not a property of the radiator.

A heat loss calculation answers one question: how bad is the worst hour. A fuel bill asks a different one: what does the whole season add up to.

Refrigeration keeps two questions apart, and half the mistakes in this subject come from letting them touch. How much cooling? is a CAPACITY.

Read the name and the algebra follows: kilowatts PER ton puts the tonnage underneath. This is the one figure of merit in the chapter where lower is better, and that single fact catches more errors than any unit check.

A vapour-compression cycle is four processes, and the pressure-enthalpy chart numbers the corners in the direction the refrigerant travels.

The cycle on the last page had a perfect compressor: it followed a line of constant entropy straight up the chart.

While liquid and vapour are both present, the thermometer will not move.

The units do the explaining. A kilowatt IS a kilojoule per second, so kJ/s divided by kJ/kg leaves kg/s and nothing else.

Here is the mistake this lesson exists to kill: a cooling tower does NOT reject the chiller's cooling load.

Three temperatures describe a cooling tower and two differences grade it.

A cooling tower cools by throwing water away. That is not a flaw, it is the mechanism — and it means the water chemistry inside the loop is a bookkeeping problem with exactly three exits.

A boiler horsepower is not a horsepower. It is a POWER — 33,475 BTU per hour, or 9.81 kW — which is more than THIRTEEN times the 746 W that the same word means on a motor nameplate.