Cooling Load Analysis and Air Diffusion Design for a Single Storey Office

Sample Answer

Cooling Load Analysis and Air Diffusion Design for a Single Storey Office

Introduction

Managing thermal comfort in small commercial buildings depends on understanding how heat enters a space and how mechanical systems respond to that load. The CLTD, SCL and CLF method remains one of the most widely used procedures for estimating cooling loads because it breaks down how solar, structural and internal gains contribute to the overall demand on an air conditioning system. This report studies a single storey office at 40°N latitude, occupied continuously throughout the day, with a single west facing external façade. The aim is to classify the relevant zone types, determine the corrected cooling load temperature differences and calculate the cooling load components at 16:00 in July. The final section designs an appropriate air diffusion system for a constant air volume arrangement in line with the dimensions provided.

Determining Zone Types

The CLTD, SCL and CLF method requires zoning because different internal gains follow different profiles over the day.

Glass Solar Zone Type

A west facing window is exposed to strongest solar radiation in the late afternoon. According to standard ASHRAE tables, this corresponds to a “West Glass Solar Load Zone”. Typical SCL values for west facing glass peak at 16:00 which aligns with this assignment’s required hour.

People and Equipment Zone Type

This office is occupied 24 hours a day, which means internal sensible gains follow a “Continuous Occupancy Zone”. People and equipment are treated as constant sensible loads whose cooling load factors depend on occupancy stability.

Lighting Zone Type

Office lighting is normally considered part of the “General Office Lighting Zone”. Since the space operates continuously, the CLF at 16:00 is close to steady state.

CLTD Correction Factor

The CLTD value from tables must be corrected for actual outdoor design temperature, indoor design temperature and for wall colour or construction type. A common correction formula is:

Corrected CLTD = CLTD(table) + (To - 29.4) - (Ti - 24)

If we assume:

Outdoor design temperature To = 35°C
Indoor design temperature Ti = 24°C
Table CLTD for west wall at 16:00 in July = 29°C

Corrected CLTD becomes:
29 + (35 - 29.4) - (24 - 24) = 34.6°C

This corrected figure is now used in all envelope transmission calculations.

Cooling Load Components at 16:00 in July

Solar Load Through the Glass

Solar heat gain is calculated as:

Q = A × SC × SCL

Assume:

Window area A = 12 m²
Shading coefficient SC = 0.75
SCL at 16:00 for west glass = 240 W/m²

Q = 12 × 0.75 × 240
Q = 2160 W

Transmission Load Through the West Window

Transmission load uses the U-value and corrected CLTD.

Q = U × A × CLTDcorr

Assume:

U = 5.7 W/m²K (single glazing)
A = 12 m²
CLTDcorr = 34.6°C

Q = 5.7 × 12 × 34.6
Q ≈ 2370 W