Sizing of Installations

Basics of Sizing Applications
Air Changes per Hour Method
Exhaust Temperature Calculation – Heat Load
Exhaust Temperature Calculation – Heat Load with Stratification
Spot Cooling

Basics of Sizing Applications

There are a number of methods determining the number of evaporative coolers required:

A ventilation system based on the number of air changes per hour
Performing a heat balance on the building to calculate exhaust temperature
Spot cooling

In order do complete any calculations both the temperature and the flow rate out of the cooler must be calculated. The method of determining the cooler outlet temperature is described in detail in Section 17. The flow rate of the coolers is dependent on the fan curve and the air distribution system. The principle of the calculation is shown below.

The fan curve for an axial fan, as fitted to all EcoCooling coolers, has the characteristic curve as shown by the blue line. Air distribution systems have a characteristic curve as shown by the green line.

By using the design information from the air distribution system and the fan curve given in the cooler specification the actual flow rate can be established.

In this case an air flow rate of just over 14,000m 3/hr (3.8m 3/s) is achieved at a static pressure of 150Pa

Specialist advice must be taken to establish the actual performance of the system.

Air Changes per Hour Method

This method calculates the number of coolers required based on the number of air changes per hour with a given working volume.

The number of air changes per hour required is based partially on experience and partially on the typical values shown on the table in the next part of this document.

The working volume is that underneath the discharge of the plenum chambers as shown by the blue shaded area in the diagram below.

Air changes per hour

Example calculation:

A bakery is 20m x 24m and it is proposed to fit ECP plastic external units with a minimum plenum discharge height of 3.5m.

Volume of working area of the building: 20 x 24 x 3.5 = 1680m 3

Target air changes per hour: 30

Target air flow per hour: 30 x 1680 = 50,400m 3/hr

Air flow rate of ECP16000 14,000 m 3/hr@150Pa

Nominal number of coolers required 50,400 / 14,000 = 3.6 = 4 coolers

Therefore the proposal would be 4 ECP16000 coolers together with balanced extract to maintain a small positive pressure.
Ventilation Rates with Air Velocities

Location Air Changes Per Hour

Assembly Halls 4 – 8
Bakeries 20 – 30
Banks 4 - 8
Bathrooms 6 - 10
Bedrooms 2 - 4
Billiard Rooms * 6 - 8
Boiler Rooms 15 - 30
Cafes and Coffee Bars 10 - 12
Canteens 8 - 12
Cellars 3 - 10
Changing Rooms - Main area 6 - 10
Changing Rooms - Shower area 15 - 20
Churches 1 - 3
Cinemas and Theatres * 10 - 15
Club rooms 10 - 12
Compressor rooms 10 - 20
Conference rooms 8 - 12
Dance halls 8 - 12
Dental surgeries 12 - 15
Dye works 20 - 30
Electroplating shops 10 - 12
Engine rooms 15 – 30
Entrance Halls & Corridors 3 - 5
Factories and Workshops 8 - 10
Foundries 15 - 30
Garages (Showrooms) 6 - 8
Glasshouses 25 - 60
Gymnasiums 6 min
Hairdressing Salons 10 - 15
Hospitals - Sterilising 15 - 25
- Wards 6 - 8
Kitchens - Domestic 15 - 20
Commercial 20 -30
Laboratories 6 - 15
Launderettes 10 - 15
Laundries 10 - 30
Lavatories 6 - 15
Lecture theatres 5 - 8
Libraries 3 - 5
Living rooms 3 - 6
Mushroom Houses 6 - 10
Offices 6 - 10
Paint shops (not cellulose) 10 - 20
Photo & X-ray darkrooms 10 - 15
Public house bars 10 - 15
Recording studios 10 - 12
Recording Control rooms 15 - 25
Restaurants 8 - 12
Schoolrooms 5 - 7
Shops and Showrooms 8 - 15
Shower baths 15 - 20
Stores & warehouses 3 - 6
Swimming baths 10 - 15
Toilets 6 - 10
Utility rooms 15 - 20
Welding shops 15 - 30

Exhaust Temperature Calculation – Heat Load

This calculation is based on taking the ambient conditions of the air, calculating the discharge temperature of the evaporative cooler, and then performing an energy balance to achieve a target exhaust temperature from the building.

Ambient temperature Ti °C
Ambient RH RH%
Discharge temperature from cooler Tc °C
Heat generated in building H KW
Target Air temperature out Te °C
Volumetric flow rate of Air V m 3/s

The temperature of the air Tc leaving the evaporative cooler is found by using the psychrometric chart using an adiabatic cooling efficiency of 85%.

The flow rate of air required to meet the target exhaust temperature is then given by:

V=1.28H/(Te-Tc)

This flow rate can then be used to select the type and numbers of coolers.

Example.

A target temperature of 27 °C is required in a building which has a heat gain of 100KW. The external conditions are 30 °C 35%RH.

Step 1 – Calculate the cooler discharge temperature using the ambient conditions:

At 30 °C 35%RH and a pad efficiency of 85% the discharge temperature from the evaporative cooler is 20.7 °C

Step 2 – Calculate the air flow rate required to maintain the target temperature:

Te=20.7°c

Tc=27.0 °C

H = 100KW

V = 0.78x100/(27.0-20.7)=78/6.3=12.4m 3/s

Therefore a total flow rate of 12.4m 3/s is required

Step 3 – Calculate the number of coolers required:

An ECP16000@150Pa delivers 14,000 / 3600 = 3.8m 3/s

Number of coolers 12.4 / 3.8 = 3.4 = 4 coolers

Exhaust Temperature Calculation – Heat Load with Stratification

The previous method can be further refined if a temperature stratification exists. If the degree of stratification is known then a thermal balance can be done to maintain a temperature at the lowest level in the building.

Ambient temperature Ti °C
Ambient RH RHa%
Heat generated in building H KW
Target Air temperature at lower level Tl °C
Temperature differential Td
Temperature at high level (exhaust) Th
Heat generated in building H KW
Volumetric flow rate of Air V m 3/s

The temperature of the air Tc leaving the evaporative cooler is found by using the psychrometric chart using an adiabatic cooling efficiency of 85%.

The flow rate of air required to meet the target exhaust temperature is then calculated using the following equation:

Air flow = 0.78xH/(Tl+Td-Ti)

Example

A target temperature of 24 °C is required in a building which has a heat load of 150KW. The ambient conditions are 32 °C 30%RH. There is a 5 °C temperature stratification in the building.

Step 1 – Calculate the discharge temperature of the evaporative cooler. (See Section 17)

At 32 °C 30%RH and a pad efficiency of 85% the discharge temperature from the evaporative cooler is 21.2 °C

Step 2 – Calculate the flow rate of air

V=(0.78 x 150)/(24+5-21.2)=117/7.8=15.0m 3/s

Step 3 – Calculate the number of coolers require

An ECP16000@150Pa delivers 14,000/3600=3.8m 3/s

Number of coolers 15.0 / 3.8 = 3.9 = 4 coolers

Spot Cooling

Spot cooling is calculated based on principles similar to the air changes per hour method. The output from a cooler together with the discharge height of the air can be used to understand the air changes per hour in a given area. A design can then be done to reflect the operating conditions.

A given volume of air discharged at a given height will give the diameter of a circular area from a given target number of air changes per hour to the following calculation.

Height of discharge of air H m
Diameter of circular area D m
Cooler flow rate V m 3/hr
Air Changes per hour A

A = (4 x V)/(3.142xDxDxH)

D = sqrt((4xV)/(3.142xAxH))

Example

What diameter circle will achieve 25 air changes per hour with an ECP16000 producing 14000 m 3/hr with a plenum discharge height of 3.5m?

V=14000

H = 3.5

A = 25

D = sqrt(4x14000)/(3.142x25x3.5) = sqrt (56000/274.9) = sqrt (203.7) = 14.2m