Lesson 6

Input parameters and units

Every yearly figure in EasyCAS — cost, energy, CO₂ — stands on a dozen numbers on one tab, and every number on the screen is shown in a unit you chose. This lesson moves those numbers one at a time and watches what follows them, so that you know which input to suspect when a result looks wrong.

Each step changes one thing, calculates, compares with the starting figures of lesson 1 and changes it back. Keep this table at hand — it is the compressor of lesson 1:

1. Know what each field is for

   The Input parameters tab holds the conditions of the whole scheme. They are stored in the .gf9 file and shared by all its scenarios.

   Field	What it sets	What follows it
   Normal temperature, Normal pressure	The reference state of a normal cubic metre: the "N" in Nm³ and NL/min.	Every Qn, and so every cost per Nm³.
   Ambient temperature	The temperature of the air in the network.	Density, so mass flow and actual flow Q.
   Critical pressure ratio of ideal nozzle	Where flow through an orifice chokes — 0.528 for air.	Nozzles and open pipes.
   Pressure of air	The pressure the air cost below was quoted at. Not the pressure the compressor makes — the calculation finds that.	New unit cost of the compressor.
   Compressed air cost	Price of a normal cubic metre at Pressure of air.	Cs.
   Hours per day, Days per year	Operating time.	Every yearly figure.
   Electricity cost	Price of a kWh.	Electricity consumption.
   Carbon intensity	Grams of CO2 per kWh of the local grid.	CO2.
   Atmospheric dew point	Moisture of the intake air.	The dew point checks of dryers and sensors (lesson 8).

   Pressures here and on the cards are gauge — above atmosphere. Hover a field and press F1 for its topic.

2. Double the operating time

   Set Hours per day to 16 and calculate. Click the compressor:

   Row	8 h	16 h	Unit
   Qn	1048.25	1048.25	NL/min
   Cs	2898.2	5796.4	€
   Electricity consumption	22293.9	44587.8	kWh
   CO2	8917.55	17835.1	kg

   The flow is a property of the pipe and does not move; everything counted per year doubles. Set the hours back to 8.

   Hours are the most abused input. A plant that runs three shifts five days a week runs 24 × 250 = 6000 hours, not 8760. The customer's own figure beats any default; a wrong one is multiplied straight into the report.

3. Change the price of electricity

   Set Electricity cost to 0.20 and calculate:

   Row	0.13	0.20	Unit
   Cs	2898.2	2898.2	€
   Electricity consumption	22293.9	14491	kWh
   CO2	8917.55	5796.4	kg

   That surprises people. Without a machine chosen for the scheme, the money comes from Compressed air cost, and the energy is worked back from it: 2898.2 € at 0.20 €/kWh buys 14 491 kWh. So the two prices must belong together — if the air cost was quoted with last year's tariff, the kWh and the CO₂ are off by the same ratio. Once a compressor is accepted in Compressor selection (lesson 9), energy comes from the machine instead. Set the price back to 0.13.

4. Move the normal state

   Some customers count a normal cubic metre at 0 °C (DIN 1343), some at 20 °C. Set Normal temperature to 0, leave Ambient temperature at 20 and calculate. Click the pipe:

   Row	Tn 20 °C	Tn 0 °C	Unit
   m	0.0208	0.0208	kg/s
   Q	149.75	149.75	L/min
   Qn	1048.25	942.8	NL/min
   Cs, compressor	2898.2	2606.7	€

   Nothing physical changed — the same kilograms leave the pipe every second, and m and Q stay where they were. A colder reference makes a normal cubic metre denser, so there are fewer of them. EasyCAS brings the flow of a nozzle or an open pipe to the normal state with the factor (Tn / Ta)^1.5, the temperature correction used for flow through an orifice: 1048.25 × (273.15 / 293.15)^1.5 = 942.8. And since the air is priced per normal cubic metre, the bill moves in the same ratio: a price quoted for Nm³ at 0 °C must be entered with Tn = 0. Set it back to 20.

   This is why EasyCAS never mixes the two scales. L/min is always actual air at the local pressure, NL/min (Nm³/h, scfm) always normal air. The capital N is deliberate: a small n would read as nano.

5. Switch the display units

   Open the Units tab. It changes only what you see: the scheme and the calculation are in SI inside. Set Pressure to psi and Air consumption to cfm. Nothing needs recalculating:

   Row	Before	After
   Pin, pipe	6 bar	87.02 psi
   Q, pipe	149.75 L/min	5.288 cfm
   Qn, pipe	1048.25 NL/min	36.53 scfm

   The flow list carries the actual scale; the normal rows follow it in their normal form — pick m3/h and Qn reads Nm³/h, pick cfm and it reads scfm. An scfm is not just a cubic foot of normal air: it is referred to the American standard state, 14.696 psia and 68 °F, not to the normal state of the scheme. With 1 bar and 20 °C in Input parameters that makes it 1.31 % smaller: 1048.25 NL/min is 37.02 cubic feet a minute of normal air, and 36.53 scfm. The System list now says Custom: the lists match none of the stored sets.

6. Save your own preset

   Set the lists back as in lesson 1, then press + left of the System list and name the set Lesson. From now on one click on Lesson restores all lists. − deletes the selected set; the built-in SI and Imperial can be neither deleted nor overwritten.

   Presets live in the settings file ed9.st next to the program — they belong to your installation, not to the scheme, and go with you to every file. Up to 64 of them.

7. Give one row a unit of its own

   American customers often want line pressure in psi but everything else metric. Click the pipe, then click the unit cell of the Pin row in the Item tab — the pointer is a hand over it — and choose psi.

   Pin is now shown in psi on every element of the scheme, while Pout and the other pressures stay in bar. The choice holds everywhere the value appears: Item tab, report, texts with references, Problems panel, scenario comparison. An input row is entered in that unit too. As on the Units tab in the same menu removes it.

   Check

   Pin of the pipe reads 87.02 psi, Pout of the compressor still reads 6 bar. Save the file, close it and open it again: the choice is stored in the scheme. Then put Pin back to As on the Units tab.

8. Decide how typed numbers are read

   The switch at the top of the Units tab, Input units, decides the unit of what you type:

   Setting	A 4 typed into Diameter means
   As output	4 of the unit shown next to it — 4 mm while Length is in mm, 4 in after you switch Length to inches.
   Original	4 of the unit written on the item's own card, whatever the display shows.

   Set it before you enter data: switching it afterwards converts what is already there. Air flow is the exception — a flow row of a card (Qn, Qc, Qmax…) always keeps the card's own value, labelled in the normal form of the Air consumption list.

   Inches. With Length in inches, sizes are written as fractions in steps of 1/16: 2_1/2 is two and a half inches, and a typed 2.5 becomes 2_1/2. A value in quotes, like "1/4", is a designation — a thread or a nominal size — and is never converted.

9. Set the currency

   The Currency list on the Units tab only labels the money: there are no exchange rates, and switching from € to $ does not convert 2898.2 into anything. Enter the prices in Input parameters in the currency you choose. The currency is stored in the scheme, so a file made for a customer in the US keeps its dollars on any computer.

When it does not work

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