When would a system be designed with chilled water as the cooling medium rather than direct expansion equipment?

Normally chilled water system has been preferred where the height is too much or building need multi functional options as well as long life system.

DX are normally used in small commercial buildings with tenants or small residential units.

If the demand load is very huge for example1000s of tonnage then chilled water is used.

Chw System is adopted where cooling demand is vey high. Example : Commercial High Rise Towers. Although the initial cost of setup is very high, the running & maintenance cost is average for a chilled water system.

DX systems is recommended for use where cooling demand is low to moderate.

Since the more the demand, the more is the requirement of compressor of VCR system which in turn requires more demand for Electricity. (Compressors consume a high rate of power for running)

Every system has its own advantage & disadvantage.

But If it were a commercial high rise tower I would recommend for a CHW system rather than DX system which is good decision on a long term run basis.

A chilled water system is considered when there is requirement to handle very large tonnage. The cooling effect can be produced at a remote placed and the desired effect can be obtained at the required area too. If the client is considered about the initial investment, go with DX system and provide mechanical floors in between. If the client is concerned about the future running costs and maintenance costs consider the chilled water system.

There are several considerations:

• If the air conditioning system, no matter how small, was to be designed as a conventional displacement system then chilled water, with its inherent infinitely variable off coil temperatures, would generally be required.
• Direct expansion systems have considerably less capital costs on small to medium sized installations. However, the capital costs of both types of systems become comparative after about a cooling load of about1000kW. (This figure varies greatly depending on the site layout.)
• Chilled water should be used for systems where flexibility is required i.e. where partitions and room layouts are to be altered regularly.

PDH. Online Course M149 (4 PDH)

HVAC Design Aspects:

Choosing A Right System

Instructor: A. Bhatia, B.E.

Typical Applications of Central HVAC System

*Office Buildings (low rise)

VAV; or CV in the core, and hydronic at perimeter

*Office Buildings (high-rise)

Central CV system for core and VAV or hydronic at perimeter

*Department Stores

Multiple CV or VAV air handlers

*Universities

CV, VAV or combined air-water systems at each building

*Schools

CV or VAV air handlers serving individual common areas, and hydronic or combined air-water systems in classrooms

*Hospitals

Separate CV systems for critical areas; CV or VAV for common areas; hydronic and combined air-water in patient rooms

*Hotels

VAV for common areas like lobbies, restaurants, ball rooms & banquets; fan-coil units in guest rooms for individual temperature and humidity control Assembly

*Theatres

Multiple VAV air handlers

*Libraries, Museums

Multiple CV air handlers, with precise humidity and temperature control

Typical Applications of Central HVAC System

*Office Buildings (low rise)

*Office Buildings (high-rise)

*Department Stores

*Universities

*Schools

*Hospitals

HOTELS

*Theatres

*Libraries, Museums

Basically Chilled water system is used for High Rise building and for a high tonnage load.

Direct expansion system will be used for small residential or commercial load.

There are many consideration in selecting the cooling system  such as Initial Cost, Maintenance cost, Purpose of the building, total tonnage, average load of a building etc.

It depends on the skills of an engineer to decide whether the load required by the building is needed a chillled water or a direct expansion cooling.

If the thermal heat capacity is very big the chilled water system is the best but if it isnt so big then the chiller system has its function and the dx also has his.