A Lagos summer design day sits at 33 °C dry-bulb / 28 °C wet-bulb. Under-sizing here is not conservative — it is a callback. And in a market where tenants sign multi-year leases on the assumption of a stable indoor environment, callbacks compound into rent-review disputes very quickly.
The temptation to import chiller sizing methodologies from temperate climates is strong and wrong. Ambient dry-bulb of 33 °C compresses air-cooled chiller efficiency by 15–20% versus a European rating point of 27 °C. If the schedule of loads is not corrected for local ambient, the plant will underperform its own datasheet on the day it is most needed.
Redundancy, not oversizing
We prefer N+1 chiller configurations sized at 50–60% of peak load each. This gives part-load efficiency AND single-machine resilience. A three-chiller plant at 55% each can lose one machine on a design day and still meet 110% of peak — with zero tenant complaint.
Contrast this with the traditional 2 × 100% configuration common in older Lagos towers: efficient nowhere, expensive to buy, and unable to modulate below 30% without cycling. Almost every plant we have retrofitted has moved from a 2× to a 3× or 4× configuration.
Variable primary flow
Modern VPF loops with pressure-independent control valves outperform legacy primary/secondary decks by 15–20% at partial load — the operating point where the plant lives 80% of the year.
The trade-off is control complexity. VPF requires a robust BMS that can modulate chiller staging against a differential-pressure setpoint at the hydraulically remote AHU. On a project with a weak BMS, primary/secondary remains the safer choice. Do not force VPF into a plant whose controls team cannot commission it.
Cooling towers and water conservation
Water-cooled plants deliver superior efficiency but assume reliable make-up water. In parts of Nigeria that assumption breaks. We design tower-based plants with a bore-fed reservoir sized for 72 hours of make-up and a monitored blowdown regime targeting 5 cycles of concentration.
Where borehole geology or planning constraints preclude water-cooled, we specify high-efficiency air-cooled chillers with adiabatic pre-cooling pads. The uplift in Coefficient of Performance on peak days is 8–12% and the water demand is trivial next to a full cooling tower.
Ventilation and dehumidification
Peak wet-bulb of 28 °C means the latent load in fresh air is enormous. Standard cooling coils cannot condense sufficiently at typical chilled water supply temperatures. We size dedicated outdoor air systems (DOAS) with 6 °C chilled water and reheat, keeping sensible cooling on the room-side coils.
This adds capital cost and pays back through tenant comfort, reduced mould risk, and lower churn on service calls in the first two years of operation. Tenants in premium space notice humidity long before they notice temperature.
Metering and tenant billing
BTU meters at every tenant interface allow accurate energy billing and unmask underperforming zones early. We insist on ultrasonic meters with ±2% accuracy and a data logger tied to the BMS. Landlords who cannot bill accurately end up subsidising inefficient tenants; the meter is the cheapest peace of mind in the plant.
Commissioning and seasonal tuning
A chilled-water plant is commissioned twice: once at handover, and again after the first full cooling season, when actual load patterns have emerged. The seasonal tune-up almost always improves plant kW/TR by another 5–10% at negligible cost. Building this into the maintenance contract is one of the highest-return decisions a facility manager can make.
The load calculation is the whole project
Every downstream decision — chiller selection, pump sizing, pipe diameters, riser shafts, electrical demand — flows from the peak and part-load calculation. A calculation done in a temperate-climate template and lightly localised will mis-size the plant by 10–20% and mis-schedule the electrical infrastructure entirely.
We run block-load calculations in hour-by-hour dynamic simulation (IES-VE or EnergyPlus) using ASHRAE weather data corrected for Lagos or Abuja. Solar gain is the dominant driver on east- and west-facing towers with unshaded glazing, and the peak often occurs at 15:00–16:00, not at noon. Occupancy density in Grade A office is trending upward (from 12 to 8 m²/person) and internal gains from IT loads have doubled in a decade.
The output is not a single peak number but a load duration curve. That curve — showing how many hours per year the plant operates at each load band — is what drives the redundancy and staging strategy. Sizing to peak without reading the curve is how oversized, inefficient plants get bought.
Electrical infrastructure implications
A 1,200 TR chilled water plant on the grid draws roughly 900 kW at design conditions. On generator, that is a 1,100 kVA continuous rating just for the cooling plant — before pumps, tower fans, or any other building load. The electrical infrastructure decisions cannot lag the mechanical ones by more than a few weeks.
We insist on a whole-building electrical single-line concept before chiller selection is finalised, so the transformer, generator and UPS strategies inform the mechanical staging. VFD-driven chillers with soft-start reduce peak generator sizing by 20% and are worth their capital premium in every project where the grid is unreliable — which is every project.
Refrigerant strategy in a decarbonising decade
R-134a is being phased down globally. Specifying it today for a 25-year asset is a decision to retrofit within a decade. We recommend R-1233zd or R-513A for centrifugal chillers, and R-32 or R-454B for smaller air-cooled machines. Servicing supply for these refrigerants in Lagos is now dependable; the risk premium of five years ago has evaporated.
Refrigerant leak detection compliant with F-Gas equivalent regulation should be specified whether the local regulation demands it or not. Grade A tenants increasingly ask, and refrigerant charge is one of the largest embodied-carbon items in a mechanical plant.
Handover and operational readiness
The gap between a commissioned plant and an operationally ready plant is the training and documentation package. We deliver a physical O&M manual, a searchable digital equivalent tied to the BIM model, a labelled valve and equipment tag register, and a 40-hour training programme for the facilities team. The training culminates in a scenario-based competency assessment which the facility team must pass before we sign off handover.
This is expensive and it is the single item on any handover that most reliably prevents callbacks. A plant that the facility team understands is a plant that stays efficient. A plant that they do not is a plant that drifts 15% off design in the first year of operation.