HVAC Condensate Calculation — AHU Using Psychometrics We will calculate the condensate generated by an Air Handling Unit (AHU) using the psychrometric moisture-balance method, as commonly applied in HVAC energy management and CEM studies. 1. Given data Parameter Value Airflow rate 10,000 m³/h Entering air temperature 35°C Entering relative humidity 60% Leaving air temperature 13°C Leaving relative humidity 90% Operating duration 24 hours/day Atmospheric pressure 101.325 kPa (assumed) Assumption: the airflow is the actual volumetric flow at the entering-air condition, and the AHU removes moisture as the air passes over the cooling coil. 2. Calculate the humidity ratio The humidity ratio \(W\) is the mass of water vapour per kilogram of dry air. \[ W=0.621945\frac{P_v}{P-P_v} \] Where: \(P_v\) = partial pre...
HVAC Condensate Calculation — AHU Using Psychometrics
We will calculate the condensate generated by an Air Handling Unit (AHU) using the psychrometric moisture-balance method, as commonly applied in HVAC energy management and CEM studies.
1. Given data
|
Parameter |
Value |
|
Airflow rate |
10,000 m³/h |
|
Entering air temperature |
35°C |
|
Entering relative humidity |
60% |
|
Leaving air temperature |
13°C |
|
Leaving relative humidity |
90% |
|
Operating duration |
24 hours/day |
|
Atmospheric pressure |
101.325 kPa (assumed) |
Assumption: the airflow is the actual volumetric flow at the entering-air condition, and the AHU removes moisture as the air passes over the cooling coil.
2. Calculate the humidity ratio
The humidity ratio \(W\) is the mass of water vapour per kilogram of dry air.
\[
W=0.621945\frac{P_v}{P-P_v}
\]
Where:
\(P_v\) = partial pressure of water vapour
\(P\) = atmospheric pressure, 101.325 kPa
Water-vapour pressure is calculated from:
\[
P_v=RH\times P_{ws}
\]
Here, \(P_{ws}\) is the saturation vapour pressure at the measured air temperature.
Entering air: 35°C, 60% RH
Saturation vapour pressure at 35°C is approximately 5.627 kPa.
\[
P_{v1}=0.60\times5.627=3.376\text{ kPa}
\]
\[
W_1=0.621945\frac{3.376}{101.325-3.376}
\]
\[
\boxed{W_1=0.02144\text{ kg/kg dry air}}
\]
Therefore, entering-air humidity ratio = 21.44 g/kg dry air.
Leaving air: 13°C, 90% RH
Saturation vapour pressure at 13°C is approximately 1.498 kPa.
\[
P_{v2}=0.90\times1.498=1.348\text{ kPa}
\]
\[
W_2=0.621945\frac{1.348}{101.325-1.348}
\]
\[
\boxed{W_2=0.008385\text{ kg/kg dry air}}
\]
Therefore, leaving-air humidity ratio = 8.385 g/kg dry air.
3. Calculate the moisture removed
\[
\Delta W=W_1-W_2
\]
\[
\Delta W=0.02144-0.008385
\]
\[
\boxed{\Delta W=0.01305\text{ kg/kg dry air}}
\]
The AHU removes approximately 13.05 grams of moisture per kilogram of dry air.
4. Calculate dry-air mass flow rate
Because the airflow is specified in m³/h, we first convert it to dry-air mass flow.
The specific volume of moist air per kilogram of dry air is:
\[
v=\frac{0.287042\,T_K(1+1.6078W_1)}{P}
\]
Using the entering-air condition:
Absolute temperature: \(35+273.15=308.15\text{ K}\)
Humidity ratio: \(W_1=0.02144\)
Pressure: \(101.325\text{ kPa}\)
\[
v\approx0.9030\text{ m}^3/\text{kg dry air}
\]
Therefore:
\[
\dot m_{da}=\frac{10,000}{0.9030}
\]
\[
\boxed{\dot m_{da}\approx11,074\text{ kg dry air/h}}
\]
5. Calculate AHU condensate production
The moisture-balance equation is:
\[
\dot m_w=\dot m_{da}(W_1-W_2)
\]
Substitute the calculated values:
\[
\dot m_w=11,074(0.02144-0.008385)
\]
\[
\dot m_w\approx144.54\text{ kg/h}
\]
Since 1 kg of water is approximately 1 litre:
\[
\boxed{\text{Condensate}\approx144.5\text{ L/h}}
\]
6. Calculate daily condensate production
For continuous operation over 24 hours:
\[
\text{Daily condensate}=144.54\times24
\]
\[
\boxed{\text{Daily condensate}\approx3,469\text{ L/day}}
\]
FINAL ESTIMATED RESULTS
144.5
Litres per hour
3,469
Litres per day
Assumes 24-hour operation at constant airflow and constant entering/leaving air conditions.
7. Practical application in hospital facility management
This estimate can help with:
Condensate drain sizing: assess whether the drain pan and drainage system can handle the expected water flow.
Water recovery: estimate the potential quantity of condensate available for suitable non-potable reuse, subject to water-quality assessment and applicable requirements.
Energy management: understand the latent cooling load associated with moisture removal.
For reference, the estimated latent cooling load is approximately:
\[
Q_L\approx\frac{144.54\times2450}{3600}
\]
\[
\boxed{Q_L\approx98.4\text{ kW}}
\]
This represents the approximate latent heat removed through condensation, not the AHU's total cooling capacity.
Engineering note: The calculated value is a theoretical estimate. Actual condensate depends on measured airflow, coil bypass factor, operating conditions, and the air's humidity before and after the coil. For a hospital AHU, verify these conditions before using the result for final drain or water-recovery design.
Air-Conditioning Condensate Calculation with CEM
CEM = Certified Energy Manager. In HVAC energy management, condensate calculation helps estimate the water removed from air by a cooling coil, assess latent cooling load, and evaluate opportunities for condensate recovery.
The example below uses a hospital AHU operating at 10,000 m³/h, with entering air at 35°C and 60% RH, and leaving air at 13°C and 90% RH.
1. Understand the process
The cooling coil lowers the air temperature below its dew point. Water vapour then condenses on the cold coil surface, collects in the drain pan, and flows through the condensate drain.
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2. Given data and assumptions
|
Parameter |
Symbol |
Value |
|
Airflow |
\(\dot V\) |
10,000 m³/h |
|
Entering air temperature |
\(T_1\) |
35°C |
|
Entering relative humidity |
\(RH_1\) |
60% |
|
Leaving air temperature |
\(T_2\) |
13°C |
|
Leaving relative humidity |
\(RH_2\) |
90% |
|
Atmospheric pressure |
\(P\) |
101.325 kPa |
|
Operating time |
\(t\) |
24 h/day |
Assumptions: steady operation, constant entering and leaving conditions, and airflow measured at the entering-air condition.
3. Main CEM calculation formula
The fundamental equation is the moisture balance:
\[
\boxed{\dot m_w=\dot m_{da}(W_1-W_2)}
\]
Where:
\(\dot m_w\) = condensate production, kg/h
\(\dot m_{da}\) = dry-air mass flow, kg/h
\(W_1\) = entering humidity ratio, kg water/kg dry air
\(W_2\) = leaving humidity ratio, kg water/kg dry air
For water, \(1\text{ kg}\approx1\text{ litre}\).
4. Step-by-step psychrometric calculations
Step 1 — Find saturation vapour pressure
From psychrometric data or steam tables:
Temperature | Saturation vapour pressure |
|---|---|
35°C | 5.627 kPa |
13°C | 1.498 kPa |
Step 2 — Calculate actual vapour pressure
Formula:
\[
P_v=RH\times P_{ws}
\]
Entering air:
\[
P_{v1}=0.60\times5.627
=\boxed{3.376\text{ kPa}}
\]
Leaving air:
\[
P_{v2}=0.90\times1.498
=\boxed{1.348\text{ kPa}}
\]
Step 3 — Calculate humidity ratio
Formula:
\[
W=0.621945\frac{P_v}{P-P_v}
\]
Entering air:
\[
W_1=0.621945\frac{3.376}{101.325-3.376}
\]
\[
\boxed{W_1=0.02144\text{ kg/kg}}
\]
Leaving air:
\[
W_2=0.621945\frac{1.348}{101.325-1.348}
\]
\[
\boxed{W_2=0.008385\text{ kg/kg}}
\]
Convert to grams per kilogram:
Entering air = 21.44 g/kg dry air
Leaving air = 8.385 g/kg dry air
Step 4 — Find the moisture difference
\[
\Delta W=W_1-W_2
\]
\[
\Delta W=0.02144-0.008385
\]
\[
\boxed{\Delta W=0.013055\text{ kg/kg}}
\]
The AHU removes approximately 13.055 grams of water per kilogram of dry air.
5. Calculate dry-air mass flow
Because the airflow is given in m³/h, convert it into dry-air mass flow.
The specific volume of moist air per kilogram of dry air is:
\[
v=\frac{0.287042T_K(1+1.6078W_1)}{P}
\]
Where \(T_K\) is absolute temperature in kelvin.
\[
T_K=35+273.15=308.15\text{ K}
\]
Substituting the entering-air conditions:
\[
v\approx0.9030\text{ m}^3/\text{kg dry air}
\]
Then:
\[
\dot m_{da}=\frac{\dot V}{v}
\]
\[
\dot m_{da}=\frac{10,000}{0.9030}
\]
\[
\boxed{\dot m_{da}\approx11,074\text{ kg/h}}
\]
6. Calculate condensate production per hour
Now apply the moisture-balance equation:
\[
\dot m_w=\dot m_{da}\times\Delta W
\]
\[
\dot m_w=11,074\times0.013055
\]
\[
\dot m_w\approx144.5\text{ kg/h}
\]
Therefore:
\[
\boxed{\text{Condensate}\approx144.5\text{ litres/hour}}
\]
7. Calculate daily condensate
For 24-hour continuous operation:
\[
V_{\text{daily}}=144.5\times24
\]
\[
\boxed{V_{\text{daily}}\approx3,468\text{ litres/day}}
\]
Final AHU calculation
144.5
Litres/hour
3,468
Litres/day
Theoretical estimate at constant operating conditions, rounded to practical precision.
8. Calculate latent cooling load
Condensate also indicates the latent heat removed from the air.
\[
Q_L=\frac{\dot m_w h_{fg}}{3600}
\]
Where:
\(Q_L\) = latent cooling load, kW
\(\dot m_w\) = condensate, kg/h
\(h_{fg}\) = approximate latent heat of vaporization, 2,450 kJ/kg
\[
Q_L=\frac{144.5\times2450}{3600}
\]
\[
\boxed{Q_L\approx98.4\text{ kW}}
\]
This is the approximate latent load only. The AHU's total cooling load also includes sensible cooling and may include other loads.
9. How to use the result in hospital facility management
A. Drainage design
Check the drain pan, trap, pipework, slope and discharge arrangement against the expected condensate rate and applicable design standards. Final sizing should allow for actual peak conditions and manufacturer requirements.
B. Water recovery
Theoretical production is about 3.47 m³/day. Actual recoverable water will be lower or variable depending on operating hours, humidity and coil conditions. Assess water quality before reuse.
C. Energy management
Record airflow, entering and leaving temperature and RH, condensate volume, cooling-coil performance and operating hours. Trend these values to identify changes in moisture removal and energy performance.
10. Quick reference for CEM calculations
Required result | Formula |
|---|---|
Humidity ratio | \(W=0.621945P_v/(P-P_v)\) |
Moisture difference | \(\Delta W=W_1-W_2\) |
Dry-air mass flow | \(\dot m_{da}=\dot V/v\) |
Condensate per hour | \(\dot m_w=\dot m_{da}\Delta W\) |
Daily condensate | \(V_{\text{daily}}=\dot m_w\times\text{hours}\) |
Latent cooling load | \(Q_L=\dot m_w h_{fg}/3600\) |
Important engineering note: Use the actual airflow basis and the correct air conditions. If the airflow is measured at the leaving condition, or the AHU mixes return air with outdoor air, the mass-flow calculation must reflect those conditions. For design, use measured data and an appropriate safety allowance rather than relying solely on a theoretical estimate.
Please details with diagram and pictures how to calculate Air conditioning Condensate in aga Khan Hospitalwith CEM
The moisture-balance method is consistent with the psychrometric principles described in the ASHRAE Handbook and HVAC engineering guidance.
11. Application at Aga Khan University Hospital (AKUH)
For AKUH, use the same method for individual AHUs and FAHUs serving clinical areas, operating rooms, ICUs, laboratories and general hospital spaces. The actual airflow and measured coil-entering and coil-leaving conditions are essential; the example above is not a measurement of an actual AKUH unit.
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A practical AKUH monitoring sheet should record:
|
|---|
For clinical areas, condensate recovery must be assessed under hospital infection-prevention, water-quality and engineering requirements. Do not assume the collected water is suitable for clinical use or potable applications.
12. Recommended CEM performance report
Use this calculation as a baseline, then compare calculated condensate with measured condensate over the same operating period.
|
KPI |
Calculation or purpose |
|
Condensate rate |
L/h |
|
Daily condensate |
L/h × actual operating hours |
|
Monthly condensate |
Daily production adjusted for actual operating days |
|
Water recovery ratio |
Recovered litres ÷ generated litres × 100% |
|
Latent cooling load |
Condensate rate × latent heat ÷ 3,600 |
|
Measurement variance |
(Measured − calculated) ÷ calculated × 100% |
A significant difference between calculated and measured condensate should trigger checks of RH sensors, airflow, cooling-coil condition, drain leakage, and operating conditions before concluding that the AHU is underperforming.
Recommended reference: ASHRAE Handbook—Fundamentals — psychometrics and moist-air processes. The Singapore National Environment Agency's ACMV training manual also provides worked examples of AHU moisture-transfer calculations.
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