Low Energy Air Conditioner | Two Stage Evaporative Cooling Unit

Low Energy Air Conditioner | Two Stage Evaporative Cooling Unit

A TWO-STAGE EVAPORATIVE COOLING SYSTEM

Overview
Along with vapour compression and vapour absorption there emerged a new technology called Indirect evaporative cooling. Vapour compression is the refrigeration technique wherein the dry bulb temperature gets reduced along with regulated humidity. Since it involves compression the technique, typical EER  value will be around 2.5 - 3 w/w.In the case of evaporative cooling the power consumption for delivering similar cooling capacity is 70-80% lesser as the energy-consuming equipment is limited to the blower and a water circulation pump.

Evaporative cooling


Figure 1: Evaporative Cooling On Psychometry

 
Direct evaporative cooling introduces water directly into the supply airstream (usually with a spray or some sort of wetted media - Domestic Air Cooler Principle). As the water absorbs heat from the air, it evaporates and cools the air. In a direct evaporative cooling, the dry bulb temperature is lowered but the wet-bulb temperature remains unchanged. In operation, a blower pulls air through a permeable, water-soaked pad. As the air passes through the pad, it is filtered, cooled, and humidified. Temperatures get reduced but the humidity in the air will increase which makes the technology unattractive. Overcoming this drawback and appreciating the merit of low power consumption, Indirect evaporative cooling technology has got emerged.

Working
Indirect evaporative cooling is a sensible cooling process that uses evaporative cooling as the heat sink for removing sensible heat from the process air. These systems cool the primary air stream or process air stream with evaporatively cooled secondary air streams. Two streams are isolated from each other by means of a thin wall made of polymer or metal. Both air streams travel either opposite or at a right angle to each other on a continuous basis. Water is evaporated in the secondary stream, which cools the air and subsequently the wall material.

This saturated air is exhausted back to the ambient. On the other side of the wall, the process air or primary air stream losses the heat to the cooled wall and becomes cooler but remains as dry as it was to start with because no moisture was added to this stream.


                                              Figure 2: Indirect Evaporative Cooling on Psychometry
Indirect evaporative cooling


                                     

                                               Figure 3: Indirect Evaporative cooling Working

Two-Stage Evaporative Cooling Technology (Indirect Direct)

The two-stage evaporative cooling unit combines indirect with direct evaporative cooling. This is accomplished by passing air inside a heat exchanger that is cooled by evaporation on the outside. In the second stage, the pre-cooled air passes through a water-soaked pad and gets humidified as it cools. Because the air supply to the second stage evaporator is pre-cooled, less humidity is added to the air, whose affinity for the moisture is directly related to temperature. The two-stage evaporative cooling provides air that is much cooler than either a direct/indirect single-stage system can provide individually. In many cases, these two-stage systems provide better comfort than a compressor-based system, because they maintain a more favourable indoor humidity range.An advanced two-stage evaporative cooler uses 100% outdoor air and a variable speed blower to circulate cool air. Two-stage evaporative coolers can reduce energy consumption by 60 to 75 % over conventional air conditioning systems, according to the ASHRAE. Yet this relative improvement depends on location and application. These systems will offer an EER of 9 – 10 W/W and hence are considered as one of the best low energy air conditioners.

Temperature reduction achievable using Two-stage Evaporative Cooling Unit

First, calculate the dry bulb and wet bulb temperatures achievable with indirect evaporative cooling:

Temp drop achievable = (dry bulb - wet bulb) x (efficiency of indirect module)

Example: (86 - 66) x 0.7 = 14 Deg C

Achievable temp = dry bulb - temp drop achievable

Example: 86 - 14 = 72 DB

Results after stage -1, indirect cooling

Starting DB /WB: 86C / 66C

Ending DB / WB: 72C / 61.4C

Then use the dry bulb/wet bulb values from step 3 to calculate the dry bulb/wet bulb temperatures achievable with direct evaporative cooling:

Temp drop achievable: (dry bulb - wet bulb) x (efficiency of the direct module media)

Example: (72 - 61.4) x 0.9 = 9.5 Deg C

Achievable temp = dry bulb - temp drop achievable. Note that with two-stage evaporative cooling, both the dry bulb and wet bulb temperatures are reduced.

Example: 72 - 9.5 = 62.5 DB

Results after stage -2, direct cooling 

Starting DB /WB in Deg C: 72 / 61.4
Ending DB / WB in Deg C: 62.5 / 61.4

Net results after stage-1 & 2 cooling

Starting DB /WB: 86 / 66
Ending DB / WB: 62.5 / 61.4

For providing comfort at the borderline conditions of the comfort zone, in tropics, use of indirect-Direct or Two-stage Evaporative cooling unit offers a good cost-effective and eco-friendly comfort cooling alternative. Suggested temperature and humidity conditions for selection of IDEC for adaptive comfort in India should be considered as follows:
  •  Industry Applications: Temp. < 30⁰C with RH < 70% Fresh air 100%
  •  Commercial Applications: Temp. < 28⁰C with RH < 65% Fresh air 100%
  •  Hybrid Air-conditioning: Temp. < 25⁰C with RH < 60%- Fresh air 30%

Simple format IDEC systems are most suitable for the very dry and arid climate where no additional cooling systems are required to meet ASHRAE 90.1 indoor conditions. In Arid climate zone, some form of cooling is required for more than 6000 hours a year and IDEC systems meet this at the lowest capital and operating cost. For well-designed systems, investment payback period normally does not exceed more than one to two years depending on the quality of the manufactured IDEC unit.

Case Study:

The US based software solutions MNC at their project in Pune thought to go with VRF refrigeration technology for total fresh air requirement of 100000 CFM. These fresh air calculations were based on industry standards such as ASHRAE and LEED considering 10 CFM per person. They realized that to cool this huge requirement of fresh air, conventional HVAC using DX or chilled water coil would require very high investment along with huge operating expenses.

Then decided to reduce the fresh air tonnage using indirect evaporative cooling and also recover exhaust heat. After a detailed evaluation, they opted to go in for 10 nos of IDE Cooling units (ranging from 3,000 CFM to 16,000 CFM) to pre-cool and supply the required 100,000 CFM fresh air. Even though ambient temperatures reach more than 40°C, these systems consistently maintained fresh air temperature in the range of 21-26°C. The specific energy consumption of PCU-R 0.48 KW/TR, which is 40% less than a conventional air conditioner system (0.83 KW/TR is considered).For 3600 annual running hours and at Rs.6/kwh power cost the facility has achieved a savings of 8.45 Lakhs.




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