Greenhouse Dehumidification Design Data: What to Send Before Sizing Equipment

Greenhouse dehumidification design starts with project data, not with a model number. Crop water use, night temperature, screens, ventilation, CO2 strategy and airflow layout can change the required moisture-removal capacity by a large margin.

The Minimum Data Set

A useful greenhouse RFQ should describe the crop, the room and the control objective.

ASHRAE's HVAC design pathway lists psychrometrics, moist air properties and air distribution as core design concepts (ASHRAE). Greenhouse projects add another layer: the crop itself is a changing moisture source.

Separate the Operating Periods

Greenhouse load is rarely flat across 24 hours. A single daily average can hide the period that actually sizes the equipment.

For greenhouse operators, trend data is more useful than a single snapshot. If available, send 7 to 14 days of temperature, RH, dew point, ventilation position, screen status and irrigation timing.

Convert Water Use Into a Load Assumption

Evapotranspiration (ET) is evaporation plus plant transpiration (T) from the crop-and-growing-medium boundary during a stated period. ET is therefore not pure T; separate T only when evaporation (E) is independently known for the same boundary and period. FAO56 definitions and balance

ET water mass = irrigation water mass − drainage water mass − increase in water storage. Express all terms in kg water. Other flows must be zero or separately included; do not count internal returns twice.

Water storage means actual water mass within the boundary. Do not use an uncorrected change in total pot, plant or container weight: non-water material changes, harvest or additions, and a changing weighing boundary can alter total weight. A greenhouse weighing study is a bounded measurement example. Greenhouse weighing study

Assume measurements resolve this hypothetical 24-hour period: 12 kg irrigation, 3 kg drainage and all other flows zero. Positive storage change retains water; negative change releases stored water. Results:

A period average does not reveal a night peak. Ordinary irrigation records may not resolve short periods; use representative measurements before scaling. Match periods to the question; do not multiply an average by a generic factor to infer a peak. ET is a water-vapour source for analysis, not whole-greenhouse load or equipment capacity.

Use a clear assumption format:

Convert moisture rate to latent heat rate

For a stated boundary and period, a water-vapour mass rate can be expressed as a latent-heat rate. Use m_dot in kg/h and h_fg, the vaporization latent heat for the adopted phase-change state, in kJ/kg. The NIST water and steam overview explains latent heat as enthalpy of vaporization and why water properties depend on state. This calculation needs an applicable h_fg input; it does not measure a moisture source.

Latent heat rate (kW) = moisture rate (kg/h) × h_fg (kJ/kg) / 3,600. The kg units cancel, leaving kJ/h; dividing by 3,600 seconds per hour gives kJ/s, or kW. In a purely hypothetical example, 3 kg/h × 2,400 kJ/kg / 3,600 = 2 kW. Use a property value appropriate to your calculation; these illustrative inputs are not project design values. This arithmetic does not calculate dehumidifier energy use.

The result is one latent heat component. Evaporation transfers energy into the vapour's latent component; it does not create extra total energy. Greenhouse heat exchange also has other terms, including conduction and air exchange; Greenhouse Engineering describes sensible and latent forms and water vapour from evaporation and transpiration. Do not add the same latent term twice when it is already included in a total, and do not treat it as total greenhouse cooling load or electrical power. Keep the original mass-rate boundary and period: an average rate does not establish a peak.

For a worked example, see how many dehumidifiers a 5-hectare greenhouse may need. For calculator inputs, use the commercial dehumidifier sizing calculator guide.

Air-exchange moisture accounting

This exchange term excludes crop transpiration, media evaporation, condensation and equipment removal.

Dry-air basis

Define the control volume and outdoor exchange path. Use Q_ref = n_ref × V_zone, where n_ref is air changes per matching time unit and V_zone is effective controlled volume. State the airflow reference conditions; unspecified ACH is not automatically incoming flow. Internal circulation is not outdoor exchange. UF/IFAS ventilation guidance explains volume and time relationships.

For matching temperature, moisture and pressure—or the same standard reference—m_da = Q_ref/v_ref. Per ASHRAE, v is m³ mixture/kg dry air; W is kg water/kg dry air. Divide g/kg by 1,000; neither RH nor specific humidity replaces W. Equal dry-air flows need not mean equal actual volumes.

Sign and duration

For a steady, well-mixed zone with balanced dry-air flow, L = m_da × (W_outdoor − W_zone); W_zone represents mixed exhaust. Positive means inward moisture; negative means removal—the winter article uses the opposite sign. Sum matched L_i × Δt_i for varying conditions; do not independently average inputs or extend instantaneous values across a day.

Hypothetical incoming Q=800 m³/h and same-state v=0.8 m³/kg dry air, with outdoor/zone W=0.006/0.010 kg/kg, give m_da=1,000 kg dry air/h and L=−4 kg water/h: −12 kg over 3 constant hours. These are assumed values.

Account for leakage

Vent opening percentage is not airflow.

NIST infiltration guidance explains how wind, temperature difference, fans, openings and airtightness affect leakage; measurements apply to their tested weather and configuration. Use evidence-supported ranges only. Without a valid estimate, leave exchange unknown; a checkable numeric load is unavailable.

A tracer-decay record uses how concentration falls over time to estimate air exchange in a space. Total measured exchange is not automatically pure leakage: identify and exclude other outdoor-air paths before calling it leakage. If zones exchange air, concentration decay across a boundary is not automatically outdoor-air exchange.

Check whether the space was adequately mixed, whether readings from multiple positions represent the control zone, and whether the fitted interval had stable exchange and outdoor background. Account for crop, substrate or other CO2 sources and sinks when they affect the signal. With a changing background, point-by-point subtraction alone does not establish a usable straight-line fit; the report should state how dynamic background was handled and whether the fit applies. These conditions guide interpretation; the report must state its method. See the NBS tracer-decay method.

ACH50 from a blower-door test is an airtightness measure at its specified pressure, 50 Pa. It is not the leakage rate under operating weather and fan conditions. Do not convert it to an operating ACH without a justified method and matching assumptions.

A limited greenhouse research case used background-corrected decay in a nearly airtight test chamber; see the Agronomy greenhouse study. The negative of the fitted logarithmic slope, multiplied by chamber volume, gives a positive flow rate at the chamber's test state. When passing such a result to a dry-air moisture balance, retain the reported state basis; do not silently relabel it as inlet or standard volume. The case demonstrates a method in that study, not this project's measurement or a commercial-greenhouse ACH.

Record the control zone, mixing/background conditions, weather, temperatures, fan/opening state, and interval. If the test range, mixing, source or sink terms do not match the project, leave the exchange input unknown and request interpretation rather than choosing coefficients.

Never add leakage already included in measured exchange.

Negative exchange does not remove temperature, CO2, crop or operating constraints. This term alone cannot select equipment.

Combine moisture sources with outdoor-air exchange

For a well-mixed zone at steady humidity ratio, use a dry-air basis. The EnergyPlus moisture predictor-corrector reference describes the moisture-balance terms used here.

D = G + m_da × (w_out − w_target). D is positive water-vapour removal (kg water/h); G is the total internal moisture source counted once (kg water/h); m_da is dry-air flow (kg dry air/h); and w values are humidity ratios (kg water/kg dry air). Here w_out is outdoor and w_target is the zone target. Assume steady, well-mixed air, balanced outdoor dry-air flow, and zero net storage. In this example, net surface and adjacent-zone exchanges are zero.

Hypothetical example: G = 3 kg water/h, m_da = 200 kg dry air/h, w_out = 0.012 and w_target = 0.010. Outdoor-air contribution = 200 × (0.012 − 0.010) = 0.4 kg water/h; therefore D = 3 + 0.4 = 3.4 kg water/h. These inputs are assumptions; 3.4 kg water/h is not a verified equipment capacity. If humidity, storage or other exchanges change, use a full transient, multi-term balance. Count each source once and do not add leakage already included in the outdoor-air flow. The result applies only to these assumptions.

Air Distribution Is Part of the Design Data

Capacity does not solve a greenhouse humidity problem if the dry air does not reach the crop zone. Include drawings or photos that show:

1. crop rows, benches or tiers,
2. aisles and service corridors,
3. fan positions,
4. duct or air-tube routes,
5. screen or curtain movement,
6. possible equipment locations,
7. drainage route and service access.

If the project has an existing greenhouse computer, include the control interface and the signals available for dehumidifier staging.

When a greenhouse choice is still provisional, compare the decision with the missing evidence rather than with a single headline number. The capacity comparison reference explains why test conditions matter; use the table below to frame the next project check.

RFQ Checklist

Before sending an inquiry, prepare this package:

Use the General Project Information Form (Word) together with the Agriculture Project Preparation Form (Word) to organize the information you already have. Fill in known facts, mark estimates as “estimated,” and leave unknown items blank. These forms support project discussion; they do not confirm a model, performance, or quotation. You do not need to complete every field before asking a question. When ready, use the agriculture project inquiry.

Use the editable Word supplier response template to record proposed capacity, supporting conditions, interface differences and open questions against your project requirements.

Then decide whether the project needs ventilation strategy, refrigerant dehumidifiers, desiccant equipment or a hybrid approach. The agriculture dehumidification overview is the correct application entry point.