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AIR-CONDITIONING SYSTEMS & VFD PUMPS

VFD pumping systems suit air-conditioning application extremely well as they can adapt to the varying heat load and compensate automatically. This Bulletin describes the areas that should be investigated when designing air-conditioning systems to improve efficiencies and thereby running costs for these systems. As there are many varieties of system I have only looked at the overview and specific applications should be treated on a case by case basis.

TYPICAL SYSTEM

The diagram below shows a typical air-conditioning loop with the chiller producing a chilled water supply to the air handling units and the cooling tower cooling the excess chiller cooling water.

Chilled Water Loop Chiller Cooling Tower

Warm Water return

Cool water return line Cooling Tower Pump Typ 6 degC Chilled Water pump

Dump valve Typ 14 degC Chiller

The main areas that apply to cost saving come from the Chilled water reticulation pumps, Cooling tower pump and Air handling unit fans. Savings in running cost of up to 80% can be generated by use of VFD technology in these areas.

GENERAL

The physics of mass transfer and heat exchange is described by the formula:

P = m x Cp x T

P= power kW m = mass transfer kg/s Cp = 4.186 (constant) T = temperature differential degC The formula details the relationship between mass flow and temperature change. The pump within any cooling loop will input power into the system equivalent to the efficiency loss of the pump. This is transferred into heat in the pumped liquid which is undesirable in cooling water lines. When a pump operates it has an efficiency for any specific duty. The measure of efficiency is based on the formula: eff = Head x Flow 102 x power The excess power input into the pump is dispelled as noise and heat. The proportion of energy loss dispelled as heat is approximately 98% of this energy. Virtually all of the energy lost as pump efficiency goes into the pumped liquid and is dissipated into the Chilled water line. This can be very significant if the pump is not operating at the optimum duty. e.g. If a chilled water loop is operating at a variable flow rate from 5 to 40 l/s at 30m head with the water temp at 3 deg C. The table below shows the temperature increase based on heat inserted into the system due to pump efficiency at different flow rates. The pump used is a 100 x 80 x 160 15Kw. Flow rate Act Head Pump Efficiency Power used Temperature increase

l/s 1 5 10 15 20 30 40 m 41 41 41 40 39 35 30 % 10 25 46 60 70 80.5 82.5 kW 4.02 8.04 8.74 9.80 10.92 12.79 14.26 degC 0.96 0.38 0.21 0.16 0.13 0.10 0.09

The table above shows that the increase water temperature is significant when the pump is operating outside its optimum efficiency range. When the pump is down in the very low end flows the temperature increase is in excess of 30% of the required temperature.

Head

Temp Increase

1.00 0.90 0.80 0.70 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0 5 10 15 20 Flow 25 30 35 40

The temperature increase is only one side of the energy loss as the chiller will have to provide more energy to reduce the temperature caused by the pump losses. In addition to this energy loss the pump operating outside its efficiency range also costs considerably more to run at lower efficiencies as detailed in the table below.

The perfect system would only transfer enough liquid into heat exchangers to satisfy the need for heat transfer. The practicality of normal system does not allow for the optimization of the heat

transfer process as the feedback information is only used in a banded situation. The operator tells the appliances to run within specified temperature levels, which in some instance can be very broad.

Temp Control Vs VFD SYSTEM

3.50 3.00 2.50 Cost $A/hr 2.00 1.50 1.00 0.50 0.00 0 5 10 15 Flow l/s 20 25 30 Temp Control VFD System

This graph shows the extra running cost of a system controlled by temperature switches compared to a pump system controlled by a Variable Speed Drive (VFD). The VFD controller varies the pump speed according to the heat load within the system and can maintain the system within the temperature requirement very accurately and also holds the cooling water pump within its optimum efficiency range.

SUMMARY OF LOSSES

General losses Pump losses - extra running costs Temp increase - Additional chiller cost

Extra Cost Variation % compared to optimum 680 600

The losses incurred by poor control over thermal transfer lines is a significant part of the design of the systems which cam be minimized by use of smarter control based on system feedback. The optimum system for thermal lines is a VFD controlled system where the pump automatically varies in speed based on the thermal load required. The rest of this paper deals with the setting up and selection of these systems for specific parts of air-conditioning systems.

COOLING TOWER PUMPS

The cooling tower pumps take heat from the CHILLER and circulate the hot water through the Cooling Tower to dissipate the heat to the atmosphere. The cool water is then recirculated to the Chiller to remove excess heat from this appliance. The cooling tower is designed to operate at a maximum flow rate under the worst atmospheric conditions. The heat load for this cooling loop is directly proportional to :

Cool water return line Cooling Tower Pump

Chiller Cooling Tower

1. The load on the Chiller 2. Atmospheric conditions - efficiency of the cooling tower. The situation where both conditions are as per design is very rare so we have the circumstance where a major energy usage appliance only operates to design condition under 10% of its operating life.

To adjust the situation to suit both control parameters it is very simple to modify the flow or heat load on the cooling tower based on the requirements of the Chiller. This is done by varying the flow of the cooling tower water based on the temperature of the return water from the Cooling tower.

Chiller

If the chiller manufacture recommends that the Chiller operate with a cooling water temperature of 20 deg. C then the control parameter for operation should be to maintain a flow rate in the cooling water loop that provides this temperature. This can be done by speeding up the pump if the temperature is lower than the prescribed set point and slowing down the pump if the temperature is lower that the set point.

Cooling Tower Pump

Chiller Cooling Tower

SET POINT

TEMPERATURE DROPS

PUMP ACCELERATES

SET POINT

Cool water return line

TEMPERATURE RAISES

PUMP DECELERATES

This can be done easily with PID control systems on Great White and Marlin Control systems. It is typically more cost effective to Chiller use multiple pumps with lower motor sizes to OVERRIDE START / STOP CONTROL FROM CHILLER maintain a full flow range rather than using one very large pump and motor. This is due to the capacity of the system to maintain the optimum efficiency at all times. The capital costs of the smaller VFD is also significant as the cost of VFD's is based on the current draw. The Marlin system uses only one pump on the VFD so the size of the drive is only required to be the sizes of the largest motor within the system.

CHILLED WATER LINE

As with the cooling Tower Line the Chilled Water Line requirements are dependent upon the heat load generated within the building. With this part of the cooling system the Chiller has generally got STOP/START control based on a broad temperature band inputs. This allows the chilled water pump to operate in parallel to the Chiller however this type of control can be very wasteful of energy as there are three energy requirement within this line: 1. Chiller energy input to cool the water 2. Line losses 3. Building Heat load. Line heat losses ENERGY INPUT into CHILLER Building requirements

Air Handling Unit

Chilled Water Loop

Air Handling Unit

Warm Water return

Air Handling Unit

Typ 6 degC Chilled Water pump

Air Handling Unit

Dump valve Typ 14 degC Chiller

The Building heat load is a given based on the design and use of the building but the lines losses are subject to design optimization. The line losses are made up of: · · · · · Heat loss through pipes and fittings Efficiency of Air Handling Units Chiller efficiency Transfer pump efficiency Heat input from transfer pump.

The areas that can be addressed are the pump efficiency and the Heat input from the pump. The example shown for a typical chilled water line shows that the losses for this line are significant if the pump duty and operation does not match the required flow and head requirement of the airconditioning system. This will always be the case if the chilled water pump is not capable of changing is operating capacity based on the heat demand from the building. Some chillers operate on a staged system that operate smaller compressors for lower heat loads. This certainly assists in minimizing losses but the best method is full variation capacity that is achieved with variable speed technology.

COST COMPARISONS

The VFD systems operate are always a more expensive capital cost item compared to a fixed speed system. The only justification that can be bought forward to use a VFD system is that the capital cost increase is generally paid back by running cost savings within the first 8 - 12 months. This is based on theoretical calculation and supported by numerous case studies. The building industry is generally a very capital cost sensitive market so the inclusion of cost saving appliances is generally not considered by the builders who are driven by the need to maintain a low initial capital cost. It is up to building owners and tenants to drive the inclusion of the cost saving benefits that can be gained by using VFD technology.

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