How to Size a Water-Cooled Packaged Air Conditioner: Capacity, Airflow and Condenser Water
A water-cooled packaged air conditioner is often described by one number — 5 TR, 17.6 kW, 60,000 BTU/h. That number is a useful label, but it is not a selection. Two units with the same nameplate rating can perform quite differently once they are installed, depending on the air entering the coil, the duct system they push against and the temperature of the condenser water they receive.
Sizing one properly means matching five things at the same set of conditions:
1. The **cooling load** of the space, including how much of it is moisture (latent) rather than temperature (sensible).
2. The **rating conditions** — entering air temperature and humidity, and entering condenser water temperature.
3. The **air-side duty** — airflow and available external static pressure for a ducted unit, or air throw and coverage for a free-blow unit.
4. The **condenser water side** — flow rate, temperature rise, pressure drop and total heat rejection.
5. The **electrical supply** and which loads may run at the same time.
This guide walks through each one, then shows how to sanity-check a supplier's figures and what to put in a request for quotation (RFQ).
*A reliable selection matches every parameter at the same operating point.*
1. Start with the cooling load, not the nameplate
The right capacity is set by the space: heat from people, lighting, machinery, solar gain, fresh air and infiltration. A load calculation should separate:
- **Sensible load** — the heat that raises air temperature.
- **Latent load** — the moisture that has to be removed, from people, processes or humid outdoor air.
A space with high latent load (kitchens, some food processing, crowded rooms) may need more coil capacity, or a different airflow, than a dry workshop with the same total load.
Common capacity units
Suppliers and buyers in different markets quote capacity in different units. These conversions are worth keeping at hand:
| Unit | Equivalent |
| --- | --- |
| 1 TR (ton of refrigeration) | 3.517 kW = 12,000 BTU/h |
| 1 kW | ≈ 3,412 BTU/h ≈ 860 kcal/h |
| 5 TR | ≈ 17.6 kW ≈ 60,000 BTU/h ≈ 15,100 kcal/h |
Replacing existing units
In replacement projects, the existing nameplate is often used as the reference. That is a reasonable starting point, but it is worth asking two questions before copying it:
- **Has the load changed?** New machinery, different occupancy or a changed process can make the old rating too small or too large.
- **Was the old unit right in the first place?** A unit that has struggled for years is not a good template.
It also helps to keep three figures apart: the existing unit's nameplate rating, the required capacity at the project's design conditions, and the supplier's predicted capacity at those same conditions. They are not automatically the same number.
Oversizing is not a safe margin. An oversized unit tends to cycle on and off more often and may remove less moisture, because the coil does not run long enough to dehumidify effectively.
2. Fix the rating conditions before comparing capacities
A capacity figure only means something when it comes with the conditions it was calculated or tested at. For a water-cooled packaged unit, the key conditions are:
- **Entering air dry-bulb and wet-bulb temperature** at the evaporator coil — for example, 27 °C DB / 19 °C WB.
- **Entering condenser water temperature** — for example, 30 °C or 35 °C, depending on the cooling tower and local climate.
- **Condenser water flow rate.**
As a general pattern, warmer condenser water reduces cooling capacity and increases compressor input. Drier entering air shifts more of the coil's work toward sensible cooling. So a unit rated at mild conditions may deliver noticeably less in a hot climate with warm tower water.
When comparing quotes, ask each supplier for capacity at **your** design conditions, not only at their standard rating point.
3. Air side: ducted vs free-blow units
Water-cooled packaged units come in two broad air-side arrangements, and they are sized differently. Some platforms offer alternative air-discharge arrangements, but availability should be confirmed for the selected model. Confirm the required discharge direction, return-air arrangement and filter configuration before accepting a selection: matching capacity, airflow and ESP alone does not establish replacement compatibility.
Ducted units: airflow and external static pressure
For a ducted unit, two figures matter:
**Supply airflow.** A long-standing rule of thumb for comfort cooling is around 400 CFM per ton, which works out to roughly 680 m³/h per TR, or about 190–195 m³/h per kW. A 5 TR unit at 3,400 m³/h sits right on that line. Higher latent loads usually call for lower airflow per kW; dry, sensible-dominated spaces can use more. Treat the rule as a cross-check, not a design value.
**Available external static pressure (ESP).** This is the pressure the unit's fan can deliver beyond its own internal losses, to push air through ducts, grilles, diffusers, dampers and any external filters. The available ESP must be at least equal to the duct system's resistance at the design airflow. If it is lower, the unit may deliver less air than specified, which may reduce cooling capacity and affect coil operation.
ESP is quoted in different units:
| Unit | Equivalent |
| --- | --- |
| 1 mm w.g. | ≈ 9.81 Pa |
| 18 mm w.g. | ≈ 176 Pa |
| 1 in w.g. | ≈ 249 Pa |
Higher ESP generally needs more fan power. When a replacement quote shows a smaller fan motor than the existing unit at the same airflow and ESP, it is reasonable to ask for the fan curve or selection record that supports it.
Free-blow (non-ducted) cabinet units
A free-blow unit discharges air directly into the space through its own louvres or nozzles. Nordtherm's [floor-standing water-cooled packaged air conditioner](https://nordthermglobal.com/products/floor-standing-water-cooled-packaged-air-conditioner-nordtherm) is an example of this cabinet arrangement. ESP is largely irrelevant here. The questions that matter instead are:
- **Air throw** — how far the air jet reaches before it slows down.
- **Ceiling height and mounting** — in high-ceiling workshops, warm air stratifies near the roof, and the outlet arrangement decides whether cool air reaches the working zone.
- **Coverage** — how many units, and where, to avoid hot spots.
4. Condenser water side: flow, temperature rise and heat rejection
The condenser water circuit carries the unit's heat away to a cooling tower or other heat sink. Four figures define it:
- **Entering water temperature** — determined by the heat-rejection system; for cooling towers, the local design wet-bulb temperature and tower approach are key inputs.
- **Water flow rate** — m³/h per unit.
- **Water-side pressure drop** at that flow — needed to size the pump.
- **Heat rejection** — the total heat the water must carry away.
*Air-side duty and condenser-water duty must be checked at one consistent operating point.*
Heat rejection is more than cooling capacity
The condenser has to reject both the heat removed from the room and the energy put into the compressor:
**Heat rejection ≈ cooling capacity + compressor input power**
This is why a cooling tower is sized on heat rejection, not on cooling capacity. When several units share one tower, add up the heat rejection of all units that may run together.
The water-side heat balance
Flow, temperature rise and heat rejection are tied together by a simple relationship for water:
**Heat (kW) ≈ flow (m³/h) × 1.163 × temperature rise (K)**
For an illustrative operating point of 17.6 kW cooling capacity and 4.5 kW compressor input, heat rejection is about 22.1 kW. At 3.4 m³/h, the water temperature rise is about 5.6 K, so 35 °C entering water leaves at around 40.6 °C. This is a simplified water heat balance, and every figure in it must refer to the same operating point.
This check is quick and useful: if a supplier's flow, entering and leaving temperatures and heat rejection do not roughly agree, at least one figure needs explaining.
Water quality
Open cooling tower water picks up dust, scale and biological growth. Fouling on the condenser heat exchanger raises the condensing temperature, which lowers capacity and raises power input over time. Water treatment, filtration and access for cleaning belong in the design, not only in the maintenance plan.
5. Electrical supply and simultaneous loads
Confirm the supply before selection, not after:
- **Voltage, phase and frequency** — for example, 380–415 V / 3 ph / 50 Hz, or 460 V / 60 Hz. Compressors and fan motors are frequency-specific.
- **Electrical sizing data** — the manufacturer's minimum circuit current and maximum protective-device rating (MCA/MOP in North American practice), or the equivalent data required locally, together with running current.
- **Starting current** — often quoted as locked-rotor amps (LRA) for the compressor; relevant on weaker supplies or when several units start together.
If the unit includes electric heating or a humidifier, confirm two things separately: which loads are permitted to run at the same time, and whether the controls automatically prevent the others. A maximum-demand figure should state the operating modes and interlocks it depends on.
6. Optional functions: heating and humidification
Some applications — computer rooms, laboratories, some process spaces — need packaged units that also heat or humidify. If so, specify:
- **Electric heating** capacity in kW and its control stages.
- **Humidifier** type (electrode steam, resistive steam and so on), input power and steam output in kg/h.
- **Control logic** — target RH and control band, and whether humidity is controlled automatically from an RH sensor or set manually.
A temperature sensor and control-panel operation do not by themselves confirm automatic humidity control; an RH sensor and the humidity-control sequence should be specified. As an energy-balance cross-check, producing atmospheric-pressure steam from 20 °C feedwater requires approximately 0.72 kWh/kg before losses, equivalent to about 1.4 kg/h per kW of electrical input. Actual continuous output is lower once heat losses and drain/refill cycles are included, so confirm the feedwater temperature and whether the quoted output is instantaneous or continuous. This check does not apply to ultrasonic atomisation.
7. Replacing existing units: what else to check
Replacement projects bring constraints that a new installation does not have:
- **Refrigerant change.** Older units often run on R22 or R404A. A replacement will normally use a current refrigerant, which changes the compressor, charge and service requirements, but not the sizing logic.
- **Footprint and weight.** Check the plant space, access route and floor loading. A newer unit may be lighter or smaller than the old one, but the openings must still match.
- **Duct and water connections.** Supply and return flange sizes and positions, condenser water connection size and condensate drain position should be compared with what is already on site. Adapters are possible, but they add pressure drop and work.
- **Deviations.** Any difference from the existing reference — capacity, airflow, ESP, refrigerant, electrical data — should be listed clearly before the order is placed.
How to sanity-check a supplier's figures
The example below uses design conditions typical of a 5 TR ducted replacement enquiry. The figures are **illustrative design values**, not measured or guaranteed performance of any specific unit.
| Design input | Example value |
| --- | --- |
| Target cooling capacity at the stated design conditions | 17.6 kW (5 TR) |
| Entering air | 27 °C DB / 19 °C WB |
| Supply airflow | 3,400 m³/h |
| Available ESP | ≥ 176 Pa (18 mm w.g.) |
| Entering condenser water | 35 °C |
| Condenser water flow | 3.4 m³/h |
*Five consistency checks to run before accepting a supplier's selection.*
Five quick checks you can run on any quote:
1. **Airflow per kW.** 3,400 m³/h ÷ 17.6 kW ≈ 193 m³/h per kW — in line with the \~400 CFM/ton rule of thumb.
2. **Heat rejection vs cooling plus input.** If the quote gives cooling capacity and compressor input, add them. The result should be close to the stated heat rejection. If heat rejection is only slightly above cooling capacity, ask what compressor input the supplier is assuming — at 35 °C entering water, the implied efficiency may be unrealistically high.
3. **Water heat balance.** Flow × 1.163 × (leaving − entering temperature) should roughly equal heat rejection. A consistent water-side balance does not, by itself, validate the selection: cooling capacity, compressor input and heat rejection must also refer to the same operating point.
4. **Fan power vs air duty.** Moving 3,400 m³/h against 176 Pa of external resistance, plus internal losses through filter, coil and cabinet, needs a fan and motor sized for the total pressure, not just the ESP. A motor that looks small for the duty is worth a fan curve.
5. **Electrical demand.** Add up compressor, fan, heater and humidifier, then check the quoted maximum against which loads the controls allow to run together.
None of these checks replaces a factory test. They help you ask the right questions before the order, rather than after installation.
RFQ checklist for a water-cooled packaged air conditioner
Giving suppliers complete information up front is the fastest way to get comparable quotes. A useful RFQ includes:
**Duty and conditions**
- Required cooling capacity (and sensible/latent split if known)
- Entering air DB/WB at the coil
- Entering condenser water temperature and available water flow
**Air side**
- Ducted or free-blow
- Supply airflow and required available ESP (ducted)
- Discharge direction (top, front) and return arrangement
- Filter type
**Water side**
- Maximum acceptable water-side pressure drop
- Connection size and type
- Shared or dedicated cooling tower
**Electrical**
- Voltage, phase, frequency
- Any limit on starting current
**Options**
- Electric heating (kW)
- Humidification (kg/h, control method)
- Controls and communication (e.g. RS485/Modbus)
**Physical and replacement data**
- Maximum dimensions and weight
- Existing flange and connection positions
- Existing unit data, if replacing
**What to ask suppliers to return**
- Total and sensible cooling capacity at your design conditions, with the rating point and gross/net capacity basis stated
- Airflow and available ESP, with fan curve
- Water flow, pressure drop and heat rejection
- Refrigerant type and charge
- Compressor and fan data, input power, running and starting current
- Dimensions, operating weight, GA drawing and wiring diagram
- Test procedure and acceptance tolerances, if factory testing is required
- A clear list of deviations from your specification
FAQ
**How many kW is a 5 TR air conditioner?** About 17.6 kW, or 60,000 BTU/h. One ton of refrigeration equals 3.517 kW.
**What external static pressure do I need?** Enough to overcome the resistance of your duct system at the design airflow — ducts, fittings, grilles, diffusers, dampers and any external filters. It should come from a duct calculation rather than a default value. Free-blow units do not need an ESP figure.
**Should I oversize the unit to be safe?** Generally no. An oversized unit tends to short-cycle, which may reduce dehumidification and increase wear. A modest margin for known future load is reasonable; a large blanket margin is not.
**What happens if the condenser water is warmer than the design value?** Cooling capacity usually falls and compressor power rises. If the cooling tower cannot hold the design entering temperature on the hottest days, the unit may not meet the load. This is why the tower should be sized on total heat rejection at the local design wet-bulb temperature.
**Does a water-cooled packaged unit always need a cooling tower?** In the common cabinet-and-tower configuration, yes — the cooling tower is what rejects the condenser heat. Other heat sinks such as dry coolers or process water loops are possible in some designs, but they change the entering water temperature and therefore the selection.
Continue exploring
- 📖 [How to Cool a Factory Without Central Air Conditioning: 6 Practical Options](https://nordthermglobal.com/blog/how-to-cool-a-factory-without-central-ac-6-options)
- 📖 [What Is a Water-Cooled Packaged Air Conditioner? How It Works & Where It's Used](https://nordthermglobal.com/blog/what-is-a-water-cooled-packaged-air-conditioner-how-it-works-where-it-s-used)
- 📖 [Water-Cooled vs Air-Cooled Packaged AC: Which to Choose?](https://nordthermglobal.com/blog/water-cooled-vs-air-cooled-packaged-ac-which-to-choose)
- 🏭 [Floor-Standing Water-Cooled Packaged Air Conditioner](https://nordthermglobal.com/products/floor-standing-water-cooled-packaged-air-conditioner-nordtherm)
Need a second opinion on a selection?
Share your cooling load, design conditions, airflow and ESP, condenser water temperature and electrical supply, and our engineering team can review the selection with you — including ducted or free-blow arrangements, subject to model availability and engineering confirmation.
