Very basically,

DX (direct expansion) systems: The treated airstream passes through the outside (fin side) of the evaporator coil such that it is directly cooled by the expansion of refrigerant passing through the tubes of the coil.

Chilled water systems: Water is pumped through the evaporator coil - the expansion of refrigerant passing through the coil cools the water. The (same) chilled water is pumped through a separate water coil that cools the air stream.

Sorry, i didn't make myself clear. Ofcourse i know the difference bet. both systems, but i need your help to make a complete comparison (ie. from the economic point of view or for which applications one supperiors the other).
I think this is the time that this question should be made clear, now that the VRV systems are being seriously considered.

What is your cost of electrical energy as a blended kWH rate?  At $.10 per KWH you are better off with a CHW water system, if the following hold true also:

1) There is money in the budget for the higher initial cost of the CHw system.
2) There are equipment operators who are capable and have the time available to handle the more complex CHW system.

PacificSteve

Thank you PacificSteve now we're starting to get to the point. So it's now the time to consider the coparison application wise, and i mean chilled water systems Vs VRV systems (or VRF systems).
If any one could manage to compare in a pros and cons technique, it would be of a great help.   

Few more comparison between Chilled Water and VRV or DX systems

1. Closer control is available in chilled water system due to modulating the chilled water flow thru cooling coil. I do not think the modulation of refrigerant in a vrv system is not close enough.
2. VRV system has a limitation in refrigerant pipe length as well as height. Chilled water is a closed loop and does not have such limitations so long as the pump can handle the flow and dynamic head.
3 Chilled water can be used for a larger aplication on air side like AHUs, FCUs etc. VRV as far as know, is limited to FCUs only.
4 I feel the indoor air quality can be better maintained with with chilled water AHU with multi level filtration and fresh air rather than small recirculation type fcus with a limitation on filters used.
5 Chilled water can be used for a large spaces with a ducted system AHU. VRV system on air side is limited to small areas with short ducting.
6 In chw systems with ducted system we can locate the Ahu away from occupied spaces and thus minimise inconvenience due to occupied spaces during servicing. IN VRV Fcus are located within occupied spaces and a possible inconvenience to occpants during servicing.
7 Chw system can be used with the popular VAV system on air side where as VRV can only offer CAV system, of course with 3 speed fcus.
I hope the above answers your query
Regards
VVVM

Thank you VVVM, so is there anybody out there thinks that the VRV systems are supperior to chilled water systems

VRV is typically used in the following.
The hvac system in you car.
The havc systems in most houses.
Computer room units are typically dx. (keep the h2o out of the room).
Package DX rooftops on shopping ceters / malls.
Typically a job has to be over 300 tons for a chilled water system to make the economics work.

Just wanted to emphasize---the economics of a system are highly dependent on the rate of energy costs, and the presence of people who can operate / maintain the systems.

If you are paying $.03 per kWH, you would indeed need a large tonnage requirement before CHW made sense economically.  At $.15 it makes sense in much smaller tonnages.

As someone else mentioned---you also have better temperature and humidity control with CHW.  Not to mention that you are not running refrigerant lines through your building in which case a refrigerant leak can be dangerous and in extreme case, could even kill someone.  DX package rooftop unit are obviously exempt from this safety concern.

PacificSteve

I totally agree with PacificSteve regarding potential leakage problems. Cooling application below 70 deg. F will start causing trouble interms of ice formation on dx coil.

However economy of chilled water system, at and above 300 TR is totally debatable for we are using systems with even 20TR.

Regards,

Truth: Even the hardest of the problems will have atleast one simple solution. Mine may not be one.

In the UK we've seen a big move to Variable flow Refrigeration systems in the last 10 years.  VRF has now applied typically to buildings as tall as 5/6 stories. I had drawings from a designer today which had VRF specified on an 8 story building!

Yes, control is not as good but the installation and capital costs are so low in comparison, that design and Build developers go this way every time.  Maintenance and service costs are higher with VRF, but hey this is an end user issue and costs the developer nothing.

The market for DX systems in the UK is now twice as large as the chilled water system market in commercial buildings (we don't have a domestic a/c market)and this has happened even though we have guidelines in place to avoid the use of refrigerant in the space.

For me give me chilled water every time for flexiblity, maintenance, service and controlabilty.    

Dear Arabei

VRV system is not intended for high ambiant climates such as in the gulf areas, and for quite place such as hotels and hospitals, it needs a separate leak detection system and fresh are ducting in all cases. Furthermore, supply/return ducting network is still needed if you chose to install the concealed type indoor units.

You need to stick to their control system, and their are many limitations on the selection regarding the minimum/maximum indoor units you can connect to the outdoor unit. Further, you cannot make any alteration to the system in terms of adding any accessories, guages, valves, regulators or extra refregerant circuits.

The last note, and in low humidity areas, and due to the fact that the system is designed to work at a refrigerant degree of 5.5 C, you need to add humidifiers to your space.

I recently heard that it was replaced in WTC-makka because of some of above considerations.

All the best

Something else to consider:

In direct DX expansion systems you only have one heat exchanger in the low side and it is the finned evaporator. It works with some temperature diference lets say 7ºC. So te suction temperature is 7ºC lower than the air being cooled (all depending on design).

In water chilled systems you need two heat exchangers (one cooling the water and the other cooling the air) so compressor suction might be lower. For example 7ºC in the water cooler plus 5ºC in the air cooler would mean a suction pressure (temperature) 12 ºC lower than the air being cooled (all depending on design).

This means that the compressor suction will be lower in the water chiller system. Normaly eficiency (COP) goes down as suction pressure goes down.

All this means lower capacity in the water system than in the DX system (for the same compressor) or more energy consumption.

Here are some more things to consider
Chill Water

1. Better Control, Central systems have chilled water and hot water mains circulating water to the cooling and heating coils. Any number of branches can be connected to these mains to supply heating and cooling elements with water controlled by automatic valves.

2. Centrilized Maintenance: Equipment is usually within a central mechanical room and is likely to be better maintained then equipment located on the roof.

3. Diversity: The central chilled water system can be sized for the instantanious peak heat load. Rooftop units must be sized for the peak load of the areas they serve.

4. Redundancy: Central systems provide redundancy by installing multiple chillers and pumps.

5. Flexibility: Adding new chilled water circuits to the existing systm is a relitivly simple operation. An aditional chiller and pump can be added to the existing system to produce extra cooling load.

6. Possible lower installed costs for larger systems,like over 300 tons
7. Possible lower operating costs if you use a cooling tower or an evap condenser.
 DISADVANTAGES:
1. Higher installed costs for systems under 150 tons

ROOFTOP UNITS:ADVANTAGES

1.Building floor space saved since units are located on the roof.
2. Usually lower initial installed costs.

3. Simpler installation in most cases.

4. More singular responsibility because units are completely assembled and tested at the factory.
DISADVANTAGES:

1. Higher operating costs. For example, for 15-100 ton water cooled reciprocating chiller total kw/ton 1.10
 B. 20-60 ton air cooled rooftop unit Total kw/ton 1.22
 C. 80-100 ton "    "       "      "    "   kw/ton  1.18

2. Higher maintenance costs.

3. Shorter life expentancy.

4. Structural work may be required to support the units.

5. Weather proof duct work may be required.

6. Possible damage to the roof.

With VRF oil may not return to the compressor under low load conditions. Wait about 5 years till the reliability of VFR or any new technology is proven from experience. You should provide a reliable, dependable, maintainable design for your client.