Restaurant coolroom and freezer room, each on its own condensing unit

1The brief

A 120-seat restaurant. Its coolroom and freezer room stand side by side behind the kitchen, sharing one wall, each with a hinged door to the back-of-house corridor, and a condensing unit each in the yard behind. What follows is the whole brief as it is entered in Kelvin; everything after it is Kelvin’s calculation.

2Refrigerant and charge

Both systems run on R-454C, a blend of R-32 21.5 % / R-1234yf 78.5 % by mass: a low-GWP refrigerant a contractor can put in a new condensing unit today, and safety class A2L — lower toxicity, lower flammability. It is a zeotrope: at one pressure it boils and condenses over a range of temperatures (its glide), from its bubble point to its dew point. So this example states each heat exchanger’s temperature difference on a mean of that range — each unit cooler below its room, the condenser above the air — and gives each compressor duty at the dew points, as makers rate compressors for zeotropic blends. The coil’s mean is its inlet and dew point averaged; the condenser’s, its dew and bubble points averaged. Kelvin finds the pressure where each mean falls on R-454C’s own saturation data, the coil’s inlet from the cycle’s own liquid, and solves the cycle there (Design conditions). R-454C’s page has its pressure–temperature table and P–h diagram.

An A2L refrigerant’s charge is limited by AS/NZS 5149: how much a circuit may hold depends on the room its refrigerant could leak into and on who occupies the building. Kelvin doesn’t make that check. What it gives the engineer who does is each circuit’s charge in the pipework it sized and the receiver, and the volume of each room its evaporator sits in:

3Heat load

Kelvin’s heat load engine sizes each room: transmission through its six surfaces, infiltration through its door, the day’s deliveries brought down to room temperature, and what the cook, the lights and the unit cooler’s fan add; the freezer adds its defrost heater. It takes the larger of holding and pull-down, adds a safety margin and spreads the day’s load over the hours the unit runs. Each room’s design load is the duty its condensing unit is selected for.

4Doors and defrost

In a restaurant the door is the load that moves most. Kelvin works it by ASHRAE’s doorway method: the warm air that would pour through the opening with the door wide open, cut by the fraction of each hour it stands open, a flow factor, and how much of that flow the strip curtain stops. A 120-seat restaurant’s coolroom is opened every couple of minutes through service, so it is taken at the Heat Load station’s heavy traffic (30 openings an hour) and the freezer at medium (10); the engine counts that rate in every hour of the day, the quiet ones too.

The coolroom’s design load again at each of the Heat Load station’s traffic levels, with its strip curtain and with none — the heat load engine run once for each:

5Design conditions

Each condensing unit is selected at the air it must work in on a hot Melbourne afternoon: 34.8 °C on the condenser, ASHRAE 0.4 % annual cooling design dry bulb for Melbourne (ASHRAE 2013), as published in AIRAH Ecolibrium, June 2015, Table 2, the figure Kelvin's Melbourne climate profile is built from. The coil’s mean is its inlet and dew point averaged; the condenser’s, its dew and bubble points averaged (its mid-glide). The inlet is the condenser’s liquid, at the brief’s subcooling, flashed to the evaporating pressure, so Kelvin solves that pressure for the coil’s mean on R-454C’s saturation data and the cycle’s own liquid, and finds the condensing pressure whose dew and bubble points average to the condenser’s. It solves the cycle at them, the evaporating temperature at its dew point and the condensing at its bubble point, as its cycle takes a zeotrope’s. The energy analysis runs each month at Melbourne’s design-high dry bulb for that month, the condensing bubble point the same 6.7 K above the air as at design, and never below 20 °C, where head pressure control holds it.

6The two cycles

Each room has its own cycle, solved by Kelvin at the design air on, at the pressures Design conditions found: evaporating where the coil’s mean sits its TD below the room, condensing where the condenser’s mid-glide sits its TD above the air. Both are drawn on R-454C’s pressure–enthalpy diagram, the coolroom’s points marked C and the freezer’s F; point anywhere on it to read the state there.

The points and lines are Kelvin’s design results; the dome, isotherms and the readout under the diagram are R-454C’s public diagram, computed with CoolProp.

7Condensing unit duties

What each condensing unit must do, at the design air on: the capacity at its evaporating temperature, the power its compressor draws for it, and the heat its condenser rejects. Each is stated at the dew points, suction and discharge, as makers rate compressors for zeotropic blends, so the figures read straight against a maker’s table. They are duties, not a model: the equipment catalogues are the engineer’s own, so the unit is chosen from a maker’s selection data against these figures. Brand-neutral.

8Pipe sizing

Each system’s suction and liquid lines run from its condensing unit in the yard, through the building’s wall and along the corridor in the ceiling space, and drop to its unit cooler. Kelvin sizes each line to its velocity band on its own flow — the Pipe Layout station’s default — and reports what each loses in saturation temperature.

Velocity bands: ASHRAE Handbook—Refrigeration Ch.1 velocity practice (suction 4.5–20, discharge 10–18, liquid 0.5–1.25 m/s). The plan marks each run as the Pipe Layout station would: the coolroom's suction line loses more than half a kelvin, which it marks as a moderate impact on the compressor (a threshold of Kelvin’s sizing code, not a cited figure). The condensing unit’s own lines, its discharge and its condenser to receiver, are inside its casing: they are given no length here, and their sizes and charge are its maker’s.

9Refrigerant charge

Each circuit’s charge in what Kelvin sized: every line at its own density, and the receiver at its liquid level. On condensing units this size most of it is in the receiver; the coil and the unit’s condenser hold the rest, from the makers’ data.

10Energy and running cost

Each unit month by month: its cycle re-solved at the month’s design-high air, its compressor and condenser fan running the hours a day its room’s load was spread over, its unit cooler’s fans their own hours (Doors and defrost) and, for the freezer, its defrost heater, at 28 c/kWh. Upper bound at design-high ambients: every hour of each month is run at that month's design dry bulb, not its mean, so expected consumption is lower.

11The issued report

In Kelvin each stage above is a station with its own record, and each record issues as a PDF report with its inputs, results and sources. Two condensing units are two designs, so each issues its own set: