ME HVAC Systems, Topic 14, Cooling Equipment

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ME 414-514 HVAC Systems, Topic 14, Cooling Equipment 1. Vapor compression cycle (VCC) → Ch. 14

2. Introduction to the absorption cycle → Ch. 14 3. Evaporative cooling → Ch. 20, Section 20.8 4. Cooling towers → Ch. 17, Section 17.7

5. Research and development on alternative cooling approaches Vapor Compression Cycle (VCC)

The ideal vapor compression refrigeration cycle is a Rankine cycle operated in reverse.

Transition 1:2 – isentropic compression Transition 2:3 – isobaric condensation Transition 3:4 – isenthalpic expansion Transition 4:1 – isobaric evaporation

Real refrigeration effects include non-constant T and P during condensation (subcooling occurs) and evaporation (superheating occurs), and compressor efficiency. Friction losses and heat loss/gain occur in all the components and plumbing.

The power you have to put in to the compressor isWin. Capacity is the rate at which

energy can be extracted,QL. The performance measure of the cycle is the coefficient of

performance. i L R W Q h h h h in energy removed energy COP   = − − = = 1 2 4 1

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In addition to cooling industrial space directly (walk-in freezers, refrigerator trucks), vapor compression cycles are used in HVAC to chill water that is then used to cool air in the air handlers. The coupling of a VCC with water/refrigerant heat exchangers is called a chiller.

Many different working fluids – refrigerants – are available (R12, R134a, R22, R410A). We are fortunate to have access to actual refrigerant properties in EES.

Several types of compressors are in common use – reciprocating, centrifugal, screw, rotary, and scroll (the scroll compressor was patented by Neils Young, an inventor who retired to Boise in the 1990s).

The work input for an “actual” polytropic compression cycle (pv = constant) is

( )         −       − = − 1 1 1 n n i o i i in p p n v np W

n is the polytropic exponent and in general it is not the ratio of specific heats. Example 17.1 compares work in using a polytropic compression analysis in comparison with isentropic compression.

Specific characteristics of different compressors are related to their performance

(reciprocating compressors: rpm, bore, stroke; centrifugal compressors: torque, rpm, rotor radius). Figure 14.12 gives actual performance curves for typical real VCC equipment. Chillers

A chiller is equipment to produce chilled water for cooling within building zones. Typically, a VCC unit with the evaporator is water coupled. Because chillers have large thermal inertia they require part load modeling when cycled to meet loads less than their capacity.

The part load ratio (PLR) of a chiller is defined as the actual cooling load at any particular time divided by the capacity at full load as rated by the manufacturer.

full cool full L act L Q Q Q Q PLR _    = = , ,

The power input to the compressor at part load is then

(

) (

)

[

]

where A, B, and C are curve fit constants from vendor data. Table 14.10 includes these for sample chillers. It is better to use PLR data from the manufacturer of the actual compressor of interest. Example 14.9 illustrates the use of actual PLR data.

14.4 A chiller uses refrigerant 22 and operates between a low-side pressure of 65 psia and a high-side pressure of 250 psia. The chiller capacity is 200 tons. Find the refrigerant flow rate, power input, and COP of this device as well as the evaporator and condenser temperatures?

P (psia) T (o_{F) h (BTU/LBm)} P-h Diagram ASHRAE or HCB tables h (BTU/LBm) EES 1 65 26.2 106.8 173.6 2 250 151.3 121.3 188.1 3 250 112.7 43.4 110.2 4 65 26.2 43.4 110.2 ṁr = 37,850 LBm/hr = 10.5 LBm/s Ẇi = 5.49 x 105 BTU/hr = 161 kW = 216 horsepower COP = 4.37 14.6

File:D:\HVAC temp\P14.4.EES 3/19/2017 7:02:36 PM Page 1 EES Ver. 10.092: #2191: For use only by students and faculty in Mechanical Engineering, Univ. of Idaho, Moscow, Idaho SOLUTION

Unit Settings: Eng F psia mass deg COP = 4.358 h1 = 173.6 [Btu/lbm] h2 = 188.1 [Btu/lb_m] h3 = 110.2 [Btu/lb_m] h4 = 110.2 [Btu/lb_m] mr = 37860 [lb_m/hr] P1 = 65 [psia] P2 = 250 [psia] P3 = 250 [psia] P4 = 65 [psia] Phigh = 250 [psia] Plow = 65 [psia] QL = 2.400E+06 [Btu/hr] s1 = 0.4192 [Btu/lb_m-R] s2 = 0.4192 [Btu/lb_m-R] s3 = 0.2835 [Btu/lb_m-R] T1 = 26.23 [F] T2 = 151.3 [F] T3 = 112.7 [F] Wi = 550673 [Btu/hr]

No unit problems were detected.

EES suggested units (shown in purple) for h[1] h[2] h[3] h[4] h_1 h_2 .

50 75 100 125 150 175 200 225 250 100 101 102 103 104

h [Btu/lbm]

P [psia]

EES solution for Problem 14.6 - an ideal vapor compression cycle

Air-Coupled Heat Pump and Central Air Conditioning Rating Standards

Heat Pumps Test Range Energy Star

Minimum SPFhp dimensionless Seasonal bin analysis 2.49

HSPFhp Btu / (W-hr) Seasonal bin analysis 8.5

EERhp Btu / (W-hr) 47 deg F 12.5

Central Air Conditioners

SEER Btu / (W-hr) Seasonal bin analysis 15

COPseasonal dimensionless Seasonal bin analysis 4.40

EER Btu / (W-hr) 95 deg F 12.5

Heat Pumps

SPFhp – Heat Pump Seasonal Performance Factor (same as COPhp,seasonal)

HSPFhp - Heat Pump Heating Seasonal Performance Factor

HSPFhp = 3.412 x SPFhp

EERhp – Energy Efficiency Ratio for a heat pump: EERhp = COP47 x 3.412

Central Air Conditioners

SEER – Seasonal Energy Efficiency Ratio

EER – Energy Efficiency Ratio for a central air conditioner: EER = COP95 x 3.412

COPseasonal – Seasonal Coefficient of Performance

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ME 414-514 HVAC Systems Topic 14 Cooling Equipment Refrigeration Cycles Absorption Cycle

In absorption chillers (AC) the compressor is replaced by a pump and a high temperature heat source. The system operates at pressures below atmospheric. (Small absorption refrigerators are common in RVs, cabins without electricity, and mobile medical units. A propane burner is a common heat source, and even the sun can be used. Large systems for HVAC are also sometimes used.) Figure 14.6 presents a schematic for an AC

showing the components of condenser, expansion valve, and evaporator in common with a VCC. But the compressor is replaced by a pump, an absorber and a generator.

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A mixture of refrigerant and absorbent is used as the working fluid (ammonia/water, and LiBr/water are the two most common working fluids). In the LiBr system, the refrigerant is the low-pressure water vapor in the evaporator and LiBr is the absorbent. In ammonia systems, ammonia is the refrigerant and water is the absorbent.

In Figure 14.6, for a H2O-LiBr system, the thermodynamic states are identified as

follows:

No. Fluid T P State Comments

1 H2O-LiBr Low Low ‘Strong’ Lots of water

2 H2O-LiBr High High ‘Weak’ Concentrated salt solution

3 Water High High Superheated vapor 4 Water Med High Saturated liquid

4’ Water Med Low Saturated liquid After expansion valve 5 Water Low Low Saturated vapor

Because it is thermal energy, not electrical power, that is input to an AC, the definition of COP differs:

𝐶𝐶𝐶𝐶𝐶𝐶𝐴𝐴𝐴𝐴 = 𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑒𝑒𝑒𝑒𝑟𝑟𝑟𝑟𝑟𝑟𝑒𝑒𝑟𝑟_{𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑖𝑖𝑒𝑒} =𝑄𝑄̇_𝑄𝑄̇𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝑔𝑔𝑒𝑒𝑔𝑔

Example 14.3 looks at a LiBr system. Referring to Figure 14.7, 𝑟𝑟̇3+ 𝑟𝑟̇2 = 𝑟𝑟̇1 total mass balance

𝑟𝑟̇2𝑋𝑋2 = 𝑟𝑟̇1𝑋𝑋1 LiBr mass balance; X refers to the mass fraction of

LiBr in the fluid

𝑄𝑄̇𝑔𝑔𝑒𝑒𝑔𝑔= 𝑟𝑟̇3ℎ3+ 𝑟𝑟̇2ℎ2− 𝑟𝑟̇1ℎ1 generator energy balance

𝑄𝑄̇𝑐𝑐𝑐𝑐𝑔𝑔𝑐𝑐 = 𝑟𝑟̇3(ℎ3− ℎ4) condenser energy balance –water only

𝑄𝑄̇𝑒𝑒𝑎𝑎𝑎𝑎= 𝑟𝑟̇5ℎ5+ 𝑟𝑟̇2ℎ2− 𝑟𝑟̇1ℎ1 absorber energy balance

𝑄𝑄̇𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑟𝑟̇5(ℎ5− ℎ4) evaporator energy balance – water only

3 Evaporative Coolers

Evaporative coolers (swamp coolers) work well in hot, dry climates. Direct evaporative coolers work by pulling outdoor air through a media kept moist by water. Water is evaporated into the air, causing sensible and latent cooling.

i w i d o d i d evap T_T T_T , , , , − − =

ε _{effectiveness of the evaporative cooler}

𝑄𝑄̇𝑎𝑎𝑒𝑒𝑔𝑔𝑎𝑎,𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝜌𝜌𝐶𝐶𝑒𝑒𝑉𝑉̇ ∆𝑇𝑇 Δ𝑇𝑇 = 𝑇𝑇𝑟𝑟𝑐𝑐𝑐𝑐𝑟𝑟− 𝑇𝑇𝑐𝑐,𝑐𝑐

A rule of thumb used for residences: ΔT = 8ºF = 4.4ºC (the temperature difference between the room set-point and the air exiting the cooler).

Evaporative coolers near Mountain Home, Idaho used to increase gas turbine efficiency. As the incoming air cools, its density increases. Hence, for the same volumetric flow rate, a greater mass of air is pulled into the turbine. Moist air also improves combustion efficiency (take ME529 Combustion and Air Pollution to learn more).

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Close up of evaporative cooler media

A criticism of direct evaporative coolers is that they add humidity to the air, which may not be desired, but can be a merit in dry desert climates.

Indirect evaporative coolers avoid this problem. They are, essentially, 2-stage coolers. In the first stage, a direct evaporative cooler cools an air stream (secondary air). The cooled air stream is used in an air-to-air heat exchanger to cool the air used in the HVAC system (primary air). An advantage of an indirect evaporative cooler is that it can use building return air to remove some of the heat added by vented light fixtures and return fans.

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The primary exit air db temperature sent to the HVAC air handler cannot be colder than the wb temperature of the secondary air coming into the evaporative cooler. The performance factor is defined as:

𝐶𝐶𝑃𝑃 = 𝑇𝑇𝑝𝑝,𝑖𝑖−𝑇𝑇𝑝𝑝,𝑜𝑜

𝑇𝑇𝑝𝑝,𝑖𝑖−𝑇𝑇𝑤𝑤,𝑠𝑠𝑠𝑠𝑠𝑠,𝑖𝑖= 𝜀𝜀ℎ𝑥𝑥𝜀𝜀𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 𝐴𝐴̇𝑚𝑚𝑖𝑖𝑚𝑚 𝐴𝐴̇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏

The effectiveness of the direct evaporative cooler is the same as defined earlier: i w i d o d i d evap T_T T_T , , , , − − = ε

The effectiveness of the indirect heat exchanger is from heat exchanger practices: 𝜀𝜀ℎ𝑥𝑥 = 𝐴𝐴̇_𝐴𝐴̇𝑏𝑏𝑏𝑏𝑏𝑏𝑏𝑏_{𝑚𝑚𝑖𝑖𝑚𝑚(𝑇𝑇}(𝑇𝑇_{𝑏𝑏,𝑖𝑖}𝑏𝑏,𝑖𝑖_−𝑇𝑇−𝑇𝑇_{𝑤𝑤,𝑖𝑖}𝑏𝑏,𝑜𝑜₎)

The capacitance rate C is the specific heat of the air stream times its mass flow rate. The capacitance rate for the building is that for the primary air. The minimum capacitance rate is from the heat exchanger design.

𝑄𝑄̇𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐 = 𝐶𝐶𝑃𝑃𝐶𝐶̇𝑎𝑎𝑐𝑐𝑐𝑐𝑔𝑔�𝑇𝑇𝑒𝑒,𝑖𝑖 − 𝑇𝑇𝑤𝑤,𝑎𝑎𝑒𝑒𝑐𝑐,𝑖𝑖�

20.15 A classroom is to be sensibly cooled with a direct

evaporative cooler (87% effectiveness) when the outside air

condition is 95

_{F dry-bulb and 67}

_{F wet-bulb temperatures.}

The room temperature is 78

_{F and the sensible cooling load is}

62,000 Btu/hr. What airflow is required to cool the

classroom? What is the relative humidity of the air being

delivered by the evaporative cooler? What is the COP of the

cooler if the fan is driven by a 3/4 hp motor? What is the

water consumption in gallons per hour?