المقارنة بين دى اكس والشيللر المائى لتكييف المصانع والوحدات الكبيرة الحجم

Comparison

of

DX

and

Chilled Water Central Air Conditioning Plants

• Introduction

The Direct Expansion (DX) and Chilled Water central air conditioning plants are both used at different places depending on the applications and size of the place to be air conditioned.

Both of them have their own advantages and disadvantages.

• 1) DX Central Air Conditioning Plants are more Efficient

In the DX type of central air conditioning plants the air used for cooling the room is directly chilled by the refrigerant in the cooling coil of the air handling unit.

Due to this heat transfer process is more efficient, since there is no middle agency involved for the heat transfer resulting in higher cooling efficiency.

In case of the chilled water system, the cooling effect from the refrigerant is first transferred to the chilled water, which is then used to chill the air used for cooling the room.

There is some loss of the cooling effect when it is being transferred from the refrigerant to the chilled water and from there to the air due to which the chilled water systems have lesser cooling efficiency.

The chilled water acts as the secondary medium for cooling the room air in air handling unit.

Further, the chilled water has to flow long distance along the whole building.

On its way it tends to get heated due to friction of flow and also due to surrounding heat absorption.

The chilled water also has to be pumped by the pump, which adds more heat to it.

Thus as the chilled water flows from the chiller to the air handling unit and again back to the chiller, apart from the heat from air, it also absorbs lots of additional heat, which leads to high increase in its temperature.

The chiller has to remove this additional heat from the water due to which its efficiency decreases or rather additional power is required to remove this additional heat.

In the DX type of central air conditioning plants the refrigerant travels only through the small distances and there is no pump involved so the additional heat absorption is less, which makes the DX plants even more efficient.

• 2) The Chilled Water Plants can be used for the Large Multi Storey Buildings

In the direct expansion types of the central air conditioning plant, the refrigerant like R22 flows through the whole air conditioning system including the air handling unit.

When the refrigerant flows in the refrigeration piping there is lots of drop in its pressure.

Due to this the length of the refrigeration tubing and the distance between the condenser and the air handling unit should be kept minimum possible.

If the air handling unit is kept at the height more than the condenser, the loss in pressure is pronounced since refrigerant travelling from the condenser to the air handling unit is in liquid state.

As the distance between the air handling unit and the condenser increases the loss in pressure also increases.

At certain point the losses may be so high that the refrigerant may not be able to reach the air handling unit, leading to complete failure of the system.

At the larger height difference there is also oil return problem from the refrigerant to the compressor.

Due to these reasons, in direct expansion type of the central air conditioning plant there is limitation on the distance between the condenser and the air handling unit.

The distance between the two cannot be too high.

This limits the application of the direct expansion type of central air conditioning systems to the small buildings or a number of rooms on the single floor.

In such cases the plant room and air handling room and the rooms to be cooled are located on the same floor.

The height difference between the condenser and the air handling units has to be quite reasonable so that they can function well.

One of the solutions to increase the capacity of the DX systems can be to increase the number of air handling units on upper floors.

But with this will be additional number of joints in the refrigerant tubing from which there will be higher chances of leakage of highly expensive refrigerant.

This leads to too many operational and maintenance problems.

One may think that we can employ compressor of very high capacity to increase the refrigerant pressure, but this will lead to highly excessive capital and running costs of the plant.

This is because we will have to install the compressor of capacity much higher than needed.

Thus the direct expansion types of the air conditioning plants can be used only for smaller buildings or various rooms on the same floor.

There are no pressure loss problems in the chilled water system.

In this system chilled water is pumped by the pump at very high pressure, which is good enough to carry it to various floors of the multi storey building.

The losses in the pressure of water are accommodated by the sufficient capacity of the pump, which has low capital and running cost.

Further, the water doesn’t carry any oil so there are no oil return problems.

In case of the chilled water system the compressor, condenser, expansion valve and the chiller are all kept at the same level in the single plant room.

There is no problem as such of the height difference between the condenser and the air handling unit since the refrigerant does not travels to the air handling unit.

The flow of the refrigerant is limited to the chiller plant.

The water chilled in the chilled flows to the various air handling units kept on different floors of the building.

The whole arrangement and the structure of the chilled water type of central air conditioning plant makes it more suitable for cooling the large multi storey buildings and even for very long distances along the same floor level.

This makes the chilled water central air conditioners more popular than the direct expansion type ones.

[img]

المقارنة بين دى اكس والشيللر المائى لتكييف المصانع والوحدات الكبيرة الحجم Direct10

[/img]

[img]

المقارنة بين دى اكس والشيللر المائى لتكييف المصانع والوحدات الكبيرة الحجم Refrig10

[/img]

[ltr]Comparison with Chiller system[/ltr]

[ltr]Chilled Water Systems have been the preferred choice for many HVAC applications in the building sector, however it is estimated that air conditioning requirements specifically in the sectors such as Shopping Malls, Hotels & Commercial Buildings will see a significant trend shift to DX-Central Systems.[/ltr]

[ltr]This is due to the flexibility, cost savings , energy efficiency and many other advantages that a DX Central system offers over a chilled water system. [/ltr]

[ltr]As a result, HVAC designers and consultants are now advocating DX central systems to their developer and building owner clients.[/ltr]

[ltr]The below comparison, with nominated parameters, high lights some of the advantages offered by a DX Central system installed in place of conventional chilled water plant.[/ltr]

[ltr]Parameter[/ltr]	[ltr]DX-Central[/ltr]	[ltr]Chiller[/ltr]
[ltr]Space[/ltr]	[ltr]Eliminates space requirement for mechanical plantroom & increases leasable floor area.[/ltr]	[ltr]Large mechanical plantroom required.[/ltr]
[ltr]Energy Cost[/ltr]	[ltr]Energy cost savings due to localized system configuration and designated unit operation on as required basis. Only the occupied spaces are conditioned.[/ltr]	[ltr]The chiller system operates regardless of load demand, ie 10% cooling demand will generate cooling call for entire plant. [/ltr]
[ltr]AMC cost[/ltr]	[ltr]DX central package units are inexpensive to maintain, reliable operational track record and faults are quickly rectified.[/ltr]	[ltr]Central plant chiller AMC costs are high due to labour intensive maintenance regimes and expensive capital components.[/ltr]
[ltr]Common Cost[/ltr]	[ltr]Limited to maintenance of cooling tower & pumps only by the landlord[/ltr]	[ltr]Landlord is responsible for the Chillers, pumps,cooling towers, hydronic systems, BMS systems and large range of specialist kit and componentry.[/ltr]
[ltr]Skilled Labour Requirements[/ltr]	[ltr]Does not require specialized, trained technicians[/ltr]	[ltr]Specialized, trained central plant chilled water technicians required for maintenance and repair.[/ltr]
[ltr]Spare Parts[/ltr]	[ltr]Small, economical and easily available, complete range of parts can be stored at site for low cost.[/ltr]	[ltr]Chiller parts are expensive, available only from the manufacturer and not ex-stock in most cases.[/ltr]
[ltr]Compressor Failure[/ltr]	[ltr]In the event of compressor failure, the single machine only is effected and the compressor is easily and quickly replaceable.[/ltr] [ltr]The entire machine can also be replaced in just 2-3 hours at low cost.[/ltr]	[ltr]Chiller compressor or motor failure results in extended downtime of several weeks and very high cost.[/ltr]
[ltr]Complete Failure[/ltr]	[ltr]Complete failure of machine only affects the local service area[/ltr]	[ltr]Complete failure of chiller can affect the whole building and the rental income in addition to significant inconvenience.[/ltr]
[ltr]Power management[/ltr]	[ltr]Multi-metering can be installed per unit allowing individual tenancies to be responsible for energy consumption outgoings.[/ltr]	[ltr]Chiller system is common for all tenants causing difficulty with running cost segregation between tenancies.[/ltr]
[ltr]Flexibility[/ltr]	[ltr]Adding or deleting to an existing DX central system for Building modifications or extensions is easily undertaken at low cost.[/ltr]	[ltr]Chilled water central plant equipment is fixed at initial design stage and cannot be easily altered.[/ltr]
[ltr]Flexibility of Starting the Building Possession.[/ltr]	[ltr]Staged developments can benefit by way of proportional cost for unit and equipment purchase in line with development construction. Owner can install the cooling tower & piping at relatively low cost and install DX machines only to occupied areas as the space is sold or leased.[/ltr]	[ltr]Staged developments must make full up front investment for all HVAC major components. No added value available for floor by floor staged leasing or selling.[/ltr]
[ltr]Capital Cost[/ltr]	[ltr]Lower cost in terms of start up capital as compared to a chiller system[/ltr]	[ltr]Higher capital cost investment upfront in the procurement of chiller to start the system.[/ltr]
[ltr]Installation[/ltr]	[ltr]Installation process is efficient and easily managed in building construction schedule without technical complications and delays. Commissioning does not require specialist skills and can be undertaken quickly and cost effectively.[/ltr]	[ltr]Chiller system installation requires high degree of technical input and may require consultant supervision. Possible construction delays. Commissioning is more elaborate with critical processes.[/ltr]
[ltr]Piping[/ltr]	[ltr]Condensor water pipework design and installation is a simple process, insulation is not required therefore no condensation risk in ceiling and roof slabs etc. No thermal loss or long term deterioration risk. Also no harmful carcinogenic insulation material in occupied space.[/ltr]	[ltr]All chilled water pipe from plant room to each machine requires insulation. Condensation and efficiency problems, resulting in thermal loss, can develop after installation or over a period of time after insulation deterioration. Chilled water system design and installation is critical for effective transportation of cooling water throughout building. Minor errors can create performance problems.[/ltr]
[ltr]Essential services power Back UP[/ltr]	[ltr]Back up power systems for equipment servicing essential areas is easily and cost effectively provided with minimum DG set for emergency operation.[/ltr]	[ltr]Any DG back up system requirements need to account for the entire power load of the HVAC plant. This can result in additional cost for duplication of HVAC units which can be run from smaller DG sets.[/ltr]
[ltr]Electrical installation and mains cabling[/ltr]	[ltr]DX units start and stop at different times reducing the electrical maximum demand calculation for the building allowing smaller mains electrical cabling. Also smaller transformers and Capacitor Banks due to no requirement for Voltage drop allowance on high Chiller Compressor start up current draw.[/ltr]	[ltr]High starting current demand requiring allowance in building electrical system design.[/ltr]
[ltr]Electrical Fluctuations[/ltr]	[ltr]DX-Central system is robust and cooperative with fluctuating and unreliable electrical power supply.[/ltr]	[ltr]More prone to failure and damage with such occurrences specifically after the initial warranty period. CVT installation is essential. Chiller unit micro-processor control systems are very susceptible to erratic power suppIy conditions.[/ltr]
[ltr]Controls/BMS/Operators[/ltr]	[ltr]DX-Central systems do not require expensive BMS or site operators for optimum system performance.[/ltr]	[ltr]Operators and BMS systems are required to operate chiller water systems. This provides operating cost additions.[/ltr]
[ltr]Efficiency[/ltr]	[ltr]The DX-Central System utilizes the most efficient form of airconditioning technology with refrigerant to air without a secondary cooling medium. [/ltr] [ltr]There is no requirement for removing the heat from large volumes of cooling water before the airconditioning process can commence. [/ltr] [ltr]The refrigerant circuit is compact and placed directly in the area to be cooled, providing maximum efficiency, less refrigerant routing and low power consumption.[/ltr]	[ltr]Two mediums used introducing many possible inefficiencies and losses. Quite often, the advertised chiller unit efficiency does not account for the entire chilled water system.[/ltr]

» عمليات العزل المائى للمبانى والخزانات
» المقارنة بين التطويف والتشتيت/flocculation and deflocculation
» طرق رفع الكربون من المرشحات الكربونية الكبيرة المفتوحة
» المقارنة بين مميزات وعيوب خطوات المعالجة المطلوبة للمياه/Water Treatment Comparison
» الشيللرات والفرق بين الشيللر التبريدى بالمياه والشيللر التبريدى بالهواء