Winter operation (HRV, Heating, Moistening, Heat loss)

The brief

A 100 % outdoor-air unit handling 1,500 m³/h (883 CFM) at a winter design temperature of −18 °C (−0.4 °F). The air must reach the room at 22 °C (71.6 °F) with a humidity ratio matching the room design state of 20 °C / 40 % RH. The chain has five nodes — outdoor air point, heat recovery, heating, steam humidification and room heat loss — and answers four questions: how much heat the recovery unit returns, how much is left for the heating coil, how much steam the humidifier consumes, and where the room state finally settles.

1. Outdoor air: −18 °C

The first node is an Air Point at −18 °C and 99 % relative humidity. That high relative humidity only looks surprising: freezing outdoor air is practically saturated, yet it holds a humidity ratio of just W = 0.76 g/kg (5.3 gr/lb) — roughly a seventh of what the room air carries. Below freezing the difference between 90 % and 99 % RH is irrelevant to the balance, because the saturation pressure itself is tiny.

The enthalpy of this state is −16.2 kJ/kg and the density 1.383 kg/m³. A volume flow of 1,500 m³/h therefore corresponds to 2,073 kg/h (4,570 lb/h) of dry air — the one quantity that stays constant along the whole chain. Volume flow, by contrast, grows with every heating step: by the end of the chain it reaches 1,755 m³/h (1,033 CFM), 17 % more than at the intake. When a fan or a duct is sized, it matters at which point in the unit the volume flow is quoted.

2. Heat recovery: plate exchanger at 75 % efficiency

A plate exchanger is dry — it transfers no moisture between the streams, so the supply air moves along a line of constant humidity ratio:

• supply: −18 °C → 10.5 °C (−0.4 °F → 50.9 °F) at W = 0.76 g/kg, relative humidity dropping to 9.7 %
• recovered capacity: 16.53 kW (56,400 Btu/h)
• exhaust: 20 °C → −4.6 °C (68 °F → 23.7 °F); the air crosses its dew point of 6.1 °C and gives up 5.82 kg/h (12.8 lb/h) of condensate
• mean surface temperature on the exhaust side: −8.5 °C (16.7 °F)

The last two lines belong together and are the point of this example: condensate forming on a below-freezing surface means frost. The app therefore raises a freezing-danger warning on this node and reports the surface temperature. In other words, the unit cannot run at the design state without frost protection — a bypass, a preheater or fan-speed control. Try dropping the efficiency to 60 %: the surface temperature rises to −2.8 °C (26.9 °F), so the risk shrinks but does not disappear. This trade-off between recovery efficiency and frosting is what governs winter operation.

3. Heating to 22 °C

The coil lifts the air from 10.5 °C to the required 22 °C at constant humidity ratio, using 6.67 kW (22,800 Btu/h).

The comparison is what makes the number interesting: heating 1,500 m³/h from −18 °C to 22 °C without recovery would take 23.2 kW (79,200 Btu/h). The plate exchanger covers 16.53 kW of that — 71 % — leaving less than a third for the water coil. That is the case for heat recovery, expressed as concrete numbers for one design state.

Downstream of the coil, however, the relative humidity is only 4.7 %. Heating adds no moisture, it merely stretches the saturation pressure — which is why winter indoor air is dry. The cause is not the heating itself, but how little water the outdoor air brings in.

4. Steam humidification to W = 5.83 g/kg

Saturated steam at 150 °C raises the humidity ratio to the target of 5.83 g/kg (40.8 gr/lb). That target is not arbitrary: it is exactly the humidity ratio of the room design state of 20 °C / 40 % RH.

• steam consumption: 10.5 kg/h (23.2 lb/h), heat equivalent 8.01 kW (27,300 Btu/h)
• the temperature rises only from 22 °C to 23.0 °C (73.4 °F), giving 23.0 °C / 33.3 % RH
• enthalpy rises from 24.1 to 38.0 kJ/kg

This is why steam humidification is called near-isothermal: the enthalpy of the air rises by 13.9 kJ/kg, but nearly all of that is latent heat carried in the water vapour (the enthalpy of saturated steam is 2,746 kJ/kg). The sensible rise is a single kelvin, and the process line on the chart is almost vertical.

Switch the humidifier type to water (adiabatic) and the contrast is immediate: the process follows a line of constant enthalpy, the same amount of water cools the air from 22 °C down to 9.6 °C (49.3 °F) at 78 % RH — and the coil upstream would have to make up the difference.

5. Room heat loss of 1.88 kW

The final node applies the room heat loss. The supply air covers it as sensible heat, so it cools from 23.0 °C to 19.8 °C (67.6 °F) at constant humidity ratio, and its relative humidity rises to 40.5 %.

Here the chain closes the loop. The resulting room state of 19.8 °C / 40.5 % matches the 20 °C / 40 % assumed at the start as the exhaust air entering the heat recovery unit, so the model is internally consistent. Had the end of the chain landed somewhere else, that would be a signal either to recompute the recovery with the real exhaust state, or to use an input binding and let the exhaust side of the exchanger follow the room state automatically.

Design-state balance

• recovered heat (HRV): 16.53 kW (56,400 Btu/h) — 71 % of the total heating demand
• heating coil: 6.67 kW (22,800 Btu/h)
• steam humidification: 8.01 kW (27,300 Btu/h) at 10.5 kg/h of steam
• room heat loss covered: 1.88 kW (6,420 Btu/h)
• condensate on the exhaust side: 5.82 kg/h (12.8 lb/h)

Things to try in the model

• Change the recovery efficiency to 60 % or 85 % and watch the load shift between the exchanger and the coil — and what it does to the surface temperature and the frosting risk.
• Switch the humidifier to adiabatic and compare both the direction of the process and the upstream heating it demands.
• Set a different outdoor temperature for your location — the built-in climate data offers design values for a specific site instead of a blanket −18 °C.
• Add a humidity load for the room (people, process): the room state shifts to the right, the humidification target drops, and it may disappear altogether.